How Big Of A Tankless Water Heater Do I Need

How to Select the Right Size Tankless Water Heater

Tankless water heaters are classified according to the highest temperature rise that may be achieved at a given flow rate. For this reason, in order to calculate the appropriate size of a demand water heater for your home, you must first estimate the flow rate and temperature increase that will be required for its application (either the entire house or a distant use, such as a bathroom). It is important to note that you should never attempt to save money by purchasing a tankless water heater that is undersized.

Step 1:

Make a decision on the maximum number of devices that you wish to run and the overall flow rate of those devices. Then total their flow rates together (gallons per minute). This is the intended flow rate that you’ll need for the demand water heater that you’ve purchased. Consider the following scenario: you anticipate to be able to operate a hot water faucet with a flow rate of 0.75 gallons per minute while also running a shower head with a flow rate of 2.6 gallons per minute concurrently. It would be necessary for the flow rate via the demand water heater to be at least 3.26 gallons per minute.

Step 2:

Calculate the temperature rise that is necessary. To calculate the temperature rise, subtract the entering water temperature from the desired output temperature and multiply by 100. Assume that the incoming water temperature is 50 degrees Fahrenheit unless you know better. You may rest assured that you will not undersize your tankless unit if you use the low temperature assumption). If you reside in a warm climate, the temperature of your water will most likely be significantly higher. For the majority of applications, water should be heated to around 105–115°.

Step 3:

Example of sizing: An typical shower will be between 104 and 106 degrees Fahrenheit and utilize 2.6 gallons of water. Assuming that the water temperature entering your home is 40° and that you wish to create enough hot water to run two showers at the same time, what temperature rise would you need to produce to achieve this goal? Answer:You’ll need to boost the temperature of the entering water from 40 degrees to 105 degrees. The ability to heat a minimum of 5.2 gallon of water will be necessary.

Flow Rates

Is there a limit to how much hot water you may use at once? You need to operate two showers at the same time, or a shower and a pair of sinks, or anything similar. The figure below illustrates the range of water consumption ranges as well as the typical water temperatures for a variety of fixtures. In order to determine your total simultaneous water requirements, we recommend that you use the following reference points: 2.5 gpm for showers and 1.0 gpm for bathrooms.


Tub 4.0 GPM 102°F
Shower 2.5 – 3.0 GPM 104°F
Washing Machine 2.0 GPM 120°F
Dishwasher 1.5 GPM 110°F
Kitchen Sink 1.5 GPM 110°F

Consider the following scenario: If you are taking two showers at the same time, you will require 5 gallons of hot water per minute from your tankless water heater. A shower and the washing machine would each use 4.5 gallons of water per minute from the water heater, thus you would need to turn on both at the same time. In any of these scenarios, you’ll want to make sure that the unit you choose is large enough to accommodate or surpass the amount of hot water you’ll require at the same time.

It is important to note that tankless water heaters are designed to heat only potable (drinking) water, and that the water entering a tankless device should not be pre-heated.

Other Sizing Notes

Gas tankless water heatersare able to deliver a greater temperature rise per gpm than electric versions. The majority of demand water heaters are rated for a wide range of water temperature inputs. An average flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric water heaters allows for a 70°F temperature rise in the water. Increased flow rates or decreased intake temperatures can occasionally result in a reduction in the temperature of the water at the furthest faucet.

Recommended Reading:

  • Learn how to use a washer dryer combo
  • Learn about three common misconceptions about washer dryer combos
  • Learn all you need to know about wall heaters in this comprehensive guide
  • And more. Infrared Heaters 101: Your Complete Guide to Understanding Them

About Our Team

Jeff Flowers is just a person who is plagued by a chronic case of curiosity and who frustrates everyone around him with his rambling nonsense. In his journey from beer to house living, Jeff is simply attempting to hack his way through life while also writing a few notes about his experiences along the way. You can read his rants here, or you can listen to him whine about Austin traffic on Twitter at @Bukowsky. You can also follow him on Facebook.

What Size Tankless Water Heater Do I Need? (For Family Of 2,3,4,5,6)

When it comes to the tankless water heater, one of the larger mistakes is choosing a unit that istoo weakto supply all our hot water needs.You don’t want one that is too tiny, nor do you want a tankless heater that’stoo bigand will needlessly waste energy. The size of your tankless water heater should be as appropriate as feasible for your household’s hot water requirements. What size tankless water heater do I require? In order to determine how many GPM tankless water heaters I require for the gas unit and how many kW I require for the electric unit, I must first determine how many GPM tankless water heaters I require for the gas unit.

  1. What is the greatest amount of hot water you require? It’s important to know how much water per minute (measured in gallons per minute, or GPM) a particular tankless water heater can heat up, as well as how many degrees it can heat it up by.

It is necessary to establish a preliminary estimate of our maximal hot water requirements at any given point in order to properly design the tankless water heater. From 9 p.m. to 11 p.m., most families have the greatest demand for hot water. That is the time of day when we shower, brush our teeth under a hot faucet, and perhaps even have the dishwasher on. We need to keep track of how much hot water we’re using. Here’s a handy table that shows how many GPMs are required by different types of water fixtures:

Fixture Gallons Per Minute (GPM)
Shower 2.0 – 3.0 GPM
Faucet (kitchen, bathroom) 1.0 – 2.0 GPM
Dishwasher 1.5 – 2.0 GPM
Washing Machine 2.0 – 2.5 GPM

A tankless water heater with a minimum flow rate of 5 GPM and a maximum temperature of 110 F will be required if you are having a shower (with 100 percent flow and 110 F hot water) and concurrently using two faucets (with 100 percent flow and 110 F hot water). It is possible to get anything from 2 GPM to 12 GPM of hot water using a tankless heater. How many gallons per minute do you require? The ones with a flow rate of 5-10 GPM are the most suitable for the majority of houses. As previously stated, the cost of a tankless water heater rises in direct proportion to the capacity of the unit.

Keep in mind that electric tankless hot water heaters are best suited for small water demands up to 8 GPM. If you have a larger need (8 GPM or more), you should choose one of the finest gas tankless hot water heaters available.

Difference Between Maximum Water Flow And Realistic Maximum GMPs

When comparing the specifications of different tankless heaters, you will see that they all list the maximum GPMs. When it comes down to it, the highest GMP that your tankless heater will truly reach might be far lower. What is the source of the discrepancy? Because the maximum water flow in GMP is calculated by heating water to 77 degrees Fahrenheit, The inlet temperature of the water that is currently in your pipes is quite important. For example, in south Texas, the inflow water temperature is 77 degrees Fahrenheit.

  • That represents an additional 40 degrees Fahrenheit differential that a tankless water heater must overcome.
  • Because the input temperature is 77 degrees Fahrenheit in Texas, we can really obtain 10 GPM of 110 degrees Fahrenheit water.
  • In Minnesota, on the other hand, the inlet water temperature is 37 degrees Fahrenheit.
  • You don’t come from Minnesota or Texas, do you?
  • An additional example based on the infographics shown above is as follows: If you reside in Florida (inlet temperature of 77 degrees Fahrenheit), the Rinnai RU160iP SE+ Series tankless heater will have a maximum water flow of 7.1 GPM at its maximum temperature.
  • If you reside in New York, on the other hand (with an intake temperature of 52 degrees Fahrenheit), the same tankless water heater may deliver a maximum water flow of 4.5 GMP.
  • In New York, the heater must contend with an additional 25 degrees Fahrenheit.
  • Especially with the bigger units, it’s worth noting the running costs as well.
  • Another useful piece of information about propane units is how much propane is consumed by these on-demand hot water heaters.

What Size Tankless Water Heater Do I Need For A Family Of 2, 3, 4, 5, Or 6?

When it comes to tankless water heater sizing, one of the most often asked topics is how much of a unit you need for a household of multiple people. Obviously, a tankless water heater designed for a family of three will be smaller than one designed for a family of five. But what are the specific GPM (for gas-powered engines) or kW (for electric-powered engines) figures? Because of the changing temperature of the water entering the tankless hot water heater, determining the correct size of the tankless hot water heater is difficult.

A table containing estimations may be seen below. Unfortunately, the confidence ranges are extremely wide. Those in the Northern United States will require larger units than homes in the Southern United States, for example, as follows:

Number Of Family Members: Gas Tankless Heater Size (GPM) Electric Tankless Heater Size (kW)
What size tankless water heater do I need for afamily of 2? 6-8 GPM 10-18 kW
What size tankless water heater do I need for afamily of 3? 7-9 GPM 15-23 kW
What size tankless water heater do I need for afamily of 4? 8-10 GPM 20-28 kW
What size tankless water heater do I need for afamily of 5? 9-11 GPM 25-34 kW
What size tankless water heater do I need for afamily of 6? 11+ GPM 34+ kW

These data are provided just as a point of reference. The size of your tankless water heater is determined by a number of crucial criteria, such as the temperature of the water entering the tank and the amount of hot water you use on a regular basis.

Looking At Specifications Sheets

It is common to find manufacturers specifying a maximum water flow number in GMP or a maximum electric power number in kW on specification documents for their products. The GMP number for gas-powered tankless water heaters is often found on the product label, whereas the kW number for electric tankless water heaters is found on the product label. As we’ve shown, the maximum GMP is a function of context. It is dependent on where you live in the United States (because that affects the inlet water temperature).

  1. Comparing the maximum wattage of different tankless heaters (as we have done in the table of the best tankless heaters below) allows us to determine how powerful they are in comparison.
  2. Check here to see if investing in an energy-efficient tankless water heater is truly worth it (we performed some calculations).
  3. For example, if you want to replace your current 50-gallon water heater, the first question you should ask is what size tankless water heater you need.
  4. What size tankless water heater would be appropriate for a household of five, for example.

What Size Tankless Water Heater Do I Need To Replace A 50 Gallon Water Heater? (Example1)

Here is how this issue goes: You now have a tank-style water heater that holds 30, 40, 50, or even 80 gallons of water and wish to upgrade to a tankless water heater. The most significant distinction, of course, is that a water tank provides, say, 50 gallons of hot water, but a tankless water heater provides water heating on demand. For example, during a typical 10-minute shower, you use around 10 gallons of hot water on average. Taking 3 showers, running a few of faucets, running a dishwasher, and so on will easily deplete those 50 gallons in no time.

Instead of storing hot water, the tankless heater’s strong heating exchanger warms the water as it is needed, up to a particular maximum GMP limit, depending on the model.

  • If you live in the northern region of the United States, you should have a 10 GPM gas tankless water heater or at least a 27 kW electric tankless water heater. If you live in the southern region of the United States, you should have a 7 GPM gas tankless water heater or at least an 18 kW electric tankless water heater.

As a result, Rinnai, the world’s leading manufacturer of gas tankless heaters, provides a broad range of models ranging from 7 GPM to 11 GPM: Rinnai’s gas tankless versions are available in a variety of sizes. As previously said, they are considered to be the top gas tankless water heater brand.

Please keep in mind that this is simply an approximate estimation. The prudent course of action is to get a tankless heater that is somewhat more powerful than the anticipated need. It is preferable to be safe than sorry.

What Size Tankless Water Heater Do I Need For A Family Of 5? (Example2)

If five individuals reside in the same house, they can use multiple faucets or showers at the same time. This must be taken into consideration while determining the appropriate size of a tankless water heater. Showers are the home hot water user that consumes the most hot water the fastest. 5 persons can also operate many hot water taps at the same time, as well as a dishwasher and do laundry, among other things. In summary, if you live in the northern portion of the United States, where the input water temperature is lower, you would require a 10 GPM gas tankless heater or a 27 kW electric tankless heater.

See also:  How To Tell When Hot Water Heater Is Going Bad?

For those who reside in the southern part of the country, the tankless water heater’s capacity might be lowered by up to 30 percent.

Keep in mind that, especially with larger units, tankless water heater circulation pumps can save you a significant amount of money on hot water.

You won’t have to wait for hot water to start flowing, squandering all of the cold water that would have been wasted in the meanwhile.

How Many Tankless Heaters Do I Need?

This is a rather typical topic, especially when it comes to larger homes. Here’s how it works: In the majority of situations, one tankless heater is sufficient to heat an entire house. For those who live in really large homes (2 or more bathrooms), even the largest Rinnai gas tankless water heater with a flow rate of 11 GPM will not be adequate to meet all of their simultaneous hot water demands. It makes logical in these situations to install two tankless water heaters. The most common combination is as follows: The largest gas unit (11 GPM, 199,000 BTU) and the largest electric unit The large tankless water heater meets the majority of the household’s hot water requirements.

Alternatively, you might utilize two units for different parts of the home, one for one section and another for the other section of the house.

I hope this has been of assistance.

What Size Tankless Water Heater Do I Need?

Note from the editors: We receive a commission from affiliate links on Forbes Advisor. The thoughts and ratings of our editors are not influenced by commissions. Unlike traditional tank water heaters, tankless water heaters give continuous hot water to the kitchen and bathroom. For many homes, installing tankless water heaters represents a welcome break from the deplete-heat-wait cycle associated with traditional tank-style water heater installation. However, reducing the tank also implies that the tolerance margins will be reduced to a bare minimum.

Properly sizing the tankless water heater ensures that you will never be without hot water—and that you will not be forced to purchase a system that is either too large or too expensive.

You May Also Be Interested In Tankless Water Heaters Available On Home Depot

Testimonials from customers Exceptionally well-written 2Rheem Performance Platinum 9.5 GPM Liquid Propane2 Rheem Performance Platinum 9.5 GPM Liquid Propane

Rheem Performance Platinum 9.5 GPM Liquid Propane

Customer feedback on the Rinnai RU199iP RU Model Series Exceptionally well-done3

Rinnai RU199iP RU Model Series

4Rinnai V53DeP V Model Series4Rinnai V53DeP V Model Series4

Rinnai V53DeP V Model Series

Customer feedback on the Rinnai RL75eP RL Model Series 5 stars out of 5

Rinnai RL75eP RL Model Series

(Note: All data and prices are correct as of publication and are subject to change.) If you’re considering purchasing a propane tankless water heater, you may feel overwhelmed by the number of options available. Price, customer rating, maximum GPM, heating capacity (in BTUs), and Energy Star certification were all taken into consideration while compiling this top-five ranking.

What a Tankless Water Heater Does

Traditional tank-style water heaters, in contrast to tankless water heaters, heat 40 to 50 gallons of water using a gas or electric burner to heat the water. The burner helps to maintain that temperature by turning on and off intermittently when the water temperature lowers. Maintaining the temperature of a pot of water on the stovetop by turning the burner on and off as needed is analogous to this. Tankless water heaters do not store hot water in a reserve tank like traditional water heaters.

  • They are energy efficient.
  • The majority of homes have one or two tankless water heaters, which are often positioned in the basement, mudroom, utility room, or hallway of the house.
  • Customers with tank-style heaters experience the same transient temperature difference as those who use faucet-style heaters because of the distance between the water heater and the faucet.
  • The installation of supplemental heat recirculators, which circulate the water in a continuous loop between the faucet and the heater, is an option for certain households.
  • Some tankless water heaters are equipped with built-in heat recirculation systems.

How to Calculate the Right Size of Tankless Water Heater

Temperature rise is defined as the difference between the groundwater temperature in your location during the winter, or the coldest time of year, and the recommended set temperature of your tankless water heater. To find out what the typical winter groundwater temperature in your location is, look at a map showing average winter groundwater temperatures. Alternatively, during the coldest time of year in your location, you may use a thermometer to monitor the temperature of your water as it is drawn directly from the ground (usually, at an exterior faucet).

Consequently, if your recommended tankless set temperature is 120 degrees and the coldest groundwater temperature in your location is 50 degrees, the temperature rise is 70 degrees for your tankless installation.

Determine Peak Hot Water Demand

It is the difference between the groundwater temperature in your location during the winter, or when it is the coldest, and the temperature you want your tankless water heater to operate at when it is the desired setting temperature. To determine the temperature of groundwater in your region, see a map showing average winter groundwater temperatures. Another option is to use a thermometer to check the water temperature while it is being drawn from the ground, especially if it is the coldest time of year in your location at the time (usually, at an exterior faucet).

Utilities 2 gallons per minute for the clothes washer Estimate and include the maximum number of services that might be consuming hot water at any given time, such as the following: Shower = 2 gallons per minute 1.5 gallons per minute (gpm) in the kitchen sink Dishwasher = 2 gallons per minute Total flow rate: 5.5 gpm Alternatively, in a household with a large number of individuals who need hot water: Shower = 2 gallons per minute 2 gpm in a tub 1.5 gallons per minute at the bathroom sink 1.5 gallons per minute (gpm) in the kitchen sink Dishwasher = 2 gallons per minute Total flow rate: 9 gpm

Choose a Tankless Water Heater

Tankless water heater manufacturers provide calculation tables or online calculators to assist you in determining the best model for your needs based on peak hot water demand and the temperature rise in your location. For example, if the temperature rise in your location is 60 degrees, your residence may have a peak hot water demand of 6 gpm at its highest point. This may direct you to a few of models that meet the bill in this situation. However, if the temperature rises over 40 degrees Celsius, the manufacturer may propose totally other models.

You must compare and contrast the two sets of information in order to determine the most appropriate tankless water heater for your home.

Tankless Water Heater Pros and Cons

  • When compared to huge tank models, little units take up less room. There is no need to wait for the hot water to cycle because there is continuous hot water. Because of the elimination of standby hot water, there will be no unnecessary heating of water.
  • There is no hot water stored in reserve in case the electricity goes out
  • When all aspects are taken into consideration, it does not cost less than tank models. Excessive scaling as a result of the extremely hot burner, resulting in a larger requirement for routine maintenance

Compare Quotes From Top-rated Water Heater Installers

Estimates are provided without obligation.

What Size Tankless Water Heater Do I Need?

Tankless water heaters are automated household equipment that provide hot water to your household whenever you need it. They may be installed in any location in your home. Their ability to provide an instant and endless flow of hot water, lower electricity consumption, simple installation, lower maintenance costs, longer life expectancy, tax breaks, and longer warranties are just a few of the reasons why they are becoming increasingly popular in both residential and commercial construction. The most important reason is that these heaters are meant to save you more room because they can be mounted on a tiny piece of the wall, as opposed to the traditional tank-water heaters that take up a lot of floor area.

The following criteria should be considered if you plan to install tankless models and are unclear of the size to purchase:

  • The square footage of your home
  • The pace at which water flows through your home
  • Temperatures are rising
  • The number of people in your household

In this essay, we’ll go into further depth about each of these aspects. However, before we go any further into the subject, it is critical that you understand that you should never purchase a tankless water heater that is lower in capacity than you actually want simply because of price disparities. Before we get started, let’s have a look at a brief chart.

Sizing Guide for Residential Tankless Water heater

Household Occupants Usage Capacity Required (Measured in Gallon)
Electric Gas
1-2 people Regular/ Low 30 30
High 40-50 40-50
2-3 people Regular/Low 40 40
High 50 40-50
3-4 people Regular/Low 50 40
High 80 50-75
5+ people Regular/Low 80 50
High 80+ 75

1. The Size of Your House

Tankless water heaters are available in a variety of sizes. Modern water heaters, on the other hand, have the ability to completely steam your whole home’s water supply. Inefficient versions may be obtained by purchasing smaller models, which are limited to heating the water that flows into specific bathrooms in your house. While traditional storage tank heaters can only store and heat the quantity of water that you anticipate using, tankless heaters can store and heat as much water as you are able to flow through them.

2. The Rate of Water Flow in Your House

We’ve established that tankless water heaters are different from traditional tank heaters in that they steam the water as it enters your home, rather than heating it. This implies that the rate at which water is released from your faucets is governed by the flow rate supplied by your water heater, not the other way around. Taking this element into consideration allows you to operate more than one faucet or shower at the same time without your faucet slowing down. Given that the typical flow rate of one shower head is 1.8 gallons per minute, it is possible that another shower will begin to run alongside the first shower at a rate of almost the same as the first shower.

Adding up the flow rates of the two showers will give you the flow rate that you will need for your tankless water heater.

We’ve included some typical flow rates below to assist you in estimating the flow rates of the majority of your faucets.

  • Bathroom faucet flow rates range from 0.5 to 1.0 GPM
  • Showerhead flow rates range from 1.5 to 2.0 GPM
  • Kitchen tap flow rates range from 2.0 to 6.0 GPM
  • Dishwasher flow rates range from 1.5 to 2.5 GPM
  • And laundry washer flow rates range from 2.5 to 3.5 GPM.

3. Temperature rise

Following the calculation of the flow rate of each faucet in your home, you must determine how much heat your heater must generate in order to provide you with the necessary heat in your water outlets. The needed temperature rise will be determined by taking into account the temperature of the water that enters your house and that of the water that you wish to have flow out of your tap when calculating the required temperature rise. We will suppose your home’s incoming water supply is 45°F and your shower (which is connected to your tankless heater) will deliver water at 105°F; the temperature raise necessary is 60°F.

To obtain the appropriate temperature in your shower, your heater must heat 1.8 gallons of water by 60 degrees Fahrenheit every minute, assuming that the shower flows at a rate of 1.8 GPM.

Some states are expected to have temperatures as high as 75 degrees, while others will have temperatures as low as 35 degrees, according to estimates.

If you’re not sure what it is, you may use the assumption that the temperature is 40 degrees to make sure you don’t underestimate the amount of temperature rise that will be required.

4. Your Household Size

This is yet another important consideration to bear in mind when installing a tankless water heater in your house. Your tankless water heater should be capable of managing occasional usages of hot water coming from specific taps without causing damage to the system. An additional point to keep in mind is when the most people are utilizing the service. What exactly does this imply? Every home suffers a spike in hot water use when they have visitors around, are organizing a wedding, or any other event of this nature.

Even though you may not be utilizing hot water all of the time, you must take into account the periods when your use spikes and use it as a starting point when determining your tankless water heater specs and requirements.

  • 2 showerheads with a total flow rate of 3.6 GPM
  • 2 kitchen faucets with a total flow rate of 4 GPM
  • 2 laundry machines with a total flow rate of 6 GPM

We’re looking at an estimated flow rate of around 13.6 gallons per minute, which may be higher than the average rate of flow in our area. Tankless heaters are designed to provide smooth heating operation at peak periods as well as during periods of average consumption. When sizing your tankless heater, consider the highest potential simultaneous usage.

Gas Tankless Water heater or Electric Tankless Water Heater?

Aside from the size, the other component that influences the operation of a tankless water heater is the size of the tankless water heater. For the most part, gas tankless water heaters can heat up to 5 gallons of water to 70 degrees Fahrenheit in a minute, but electric tankless water heaters can only heat 2 gallons of water to 70 degrees Fahrenheit in the same amount of time. We picked 70 degrees Fahrenheit since that is the temperature rise necessary for normal groundwater at 40 degrees Fahrenheit to reach a sweltering 110 degrees Fahrenheit.

See also:  How Often Should You Replace Water Filter In Refrigerator?

Before making a purchase, always double-check the product specs for GPM and temperature increase ratings.

Tankless Water Heater Size FAQs

You may still have some queries about the size of the tankless water heater you require; we will attempt to answer them in this section.

Which sizes last longer?

Tankless water heater longevity is mostly determined by the type of tankless water heater used rather than its size. The lifespan of a gas tankless water heater is often greater than that of an electric tankless water heater. However, the type and brand of the product have an impact on the lifespan; however, in general, gas-fired tankless heaters have a lifespan of more than twenty years, whereas electric water heaters have a lifespan of seven to ten years.

Where can I find the right model size?

You’re likely to locate all model sizes in departmental plumbing stores. For your convenience, you may want to try purchasing them online; however, make sure you carefully read the requirements to avoid any misunderstandings.

What size tankless water heater can I install myself?

Because you will require the skills of a skilled installation, this is not a do-it-yourself project.

The installation procedure is time-consuming and difficult. It entails gas connections, electrical circuits, propane units, and other such things. In such instances, the business will always recommend a tech person to customers.

What size tankless water heater is easier to maintain?

In comparison to bigger models, it seems obvious that smaller model sizes would be less difficult to keep up with. Regardless, you will want the services of a professional to do routine maintenance on your tankless water heater at regular intervals. This involves checking for leaks, inspecting the air vents, and cleaning the burner, among other things. If you reside in a hard water area, you should consider flushing your tankless water heater with vinegar every three weeks to prevent sediments from blocking the heat exchanger, especially if the tankless water heater is tiny.

What are the prices for different tankless water heater sizes?

Tankless heaters are priced differently depending on their size and model. A small gas-fired variant may be purchased for roughly $180, while larger models can be purchased for over $2,000. For $1,000, you may also acquire a model that is of medium size. Electric tankless heaters range in price from $90 to $900.


Tankless heaters are available in a variety of sizes and styles. It is possible to purchase a small gas-fired variant for as little as $180, but larger models can cost as much as $2,000. Also available for $1,000 is a model in the middle of the range. In the range of $90 to $900, electric tankless heaters may be purchased.

What Size Tankless Water Heater Do I Need? (+ Sizing Calculator)

This page may contain affiliate links, so please keep that in mind. If you purchase a product or service after clicking on one of these links, we will get a commission at no additional cost to you. See our product review method in further detail, or read our FTC affiliate disclosure for more information. Tankless water heaters allow on-demand access to an infinite supply of hot water. Furthermore, they can reduce your utility bills by more than 30% due to the elimination of standby energy losses, they are about the size of a suitcase and therefore take up less space, and they have a lifespan of up to 20+ years, which is nearly twice as long as the lifespan of a traditional tank-based unit.

A system that is too tiny will not be able to deliver enough hot water to suit the demands of your entire household, so don’t skimp on this purchase.

This is why it is critical to have the right size before making a purchase.


  • In the case of a tankless water heater, the first step is to determine the flow rate. Putting it all together
  • Step 2: Temperature increase
  • Step 3: Putting it all together Video
  • In the case of a tankless water heater, the first step is to calculate the flow rate. Step 2: The temperature begins to rise
  • Step 3: Putting it all together
  • Step 4: Video

How to Size a Tankless Water Heater

To put it another way, tankless water heaters, also known as on-demand water heaters, are rated according to their maximum output water flow rate at a given temperature rise. So, in order to properly size a tankless water heater, you must take into account two factors:

  1. How many gpm (gallons per minute) of hot water you’ll use during peak consumption periods is calculated. Keep in mind that tankless water heaters do not store water, but rather heat it as it passes through them. The needed temperature rise, which is determined by the parameters for the input water temperature and the output water temperature

What happens if you are undersized? That was a bad idea! Most water heaters will automatically reduce the supply of hot water if the demand for hot water exceeds the maximum capacity, even if it is just for a brief period of time. As a result, there will be less hot water available at each outlet, resulting in a temperature and/or pressure reduction.

Furthermore, a tankless water heater that operates at maximum capacity all of the time is more likely to fail prematurely. What happens if you go a little too far? Oversizing isn’t a major concern in this situation. The main drawback is the excessive up-front expense.

Step 1: Flow Rate – How Many GPM for a Tankless Water Heater

Begin by calculating your peak hot water use requirements. This may be accomplished by referring to the chart below. It is a list of the typical flow rates of several types of water outlets available for purchase in the United States. On the other hand, you may seek up your individual fixtures and other items on the internet or in their product manuals. Simply decide which devices you want to be able to operate at the same time and how many of them you want to be able to run at the same time.

In the case of two showers and one kitchen faucet operating at the same time, your necessary maximum water flow is as follows: 2.5 gpm plus 2.2 gpm equals 4.7 gpm.

It’s just that simple!

  • By anticipating the highest demand during peak hours, such as the morning, you can assure that you will always have hot water available, no matter what happens. An further benefit is the fact that a tankless water heater that does not have to operate at full capacity all of the time is likely to survive far longer. At the same time, with a little forethought, you will be able to drastically cut peak demand. It entails taking turns in the shower, allowing the dishwasher to do its work while everyone is away from the house, and preparing your meals before or after the showers are completed. You must obviously consider not just the amount of bathrooms in your home, but also the number of people that will be living under one roof while making this decision. Even in a home with five bathrooms, two persons can only use two showers at the same time
  • Consider the following questions: Will the water heater service your complete home or only portions of it? It is important to note that the flow rates mentioned below represent total water production, which includes both hot and cold water. As a result, showering with a 2.5-gpm shower head does not guarantee that the water will be hot during the showering session. It is more likely that you will need to mix in some cold to get the required temperature level. As a result, the real demand for hot water is slightly lower
  • It is possible to limit flow rates by installing low-flow aerators or fixtures.
Water Outlet Standard Flow Rate
Hand washing sink 0.5 – 1.5 gpm
Shower head 2.5 (2.0*) gpm
Bathroom faucet 2.2 (1.5*) gpm
Bathtub faucet 3.0 – 4.0 gpm
Kitchen faucet 2.2 gpm
Washing machine 23+ gallons per load, gpm hard to determine
Dishwasher 6 gallons per load, gpm hard to determine

*Products that are WaterSense certified Please keep in mind that older fixtures will most likely have greater flow rates. Please note that we did not include flow rates for washing machines and dishwashers as you can see in the table above. This is due to the fact that we found it quite difficult to obtain credible information on this topic. Some sources state 2 to 3 gpm, while others state 1.5 gpm. Instead, you might run each appliance independently and keep an eye on the timer and your water meter for any anomalies.

As a matter of fact, to get a more scientific perspective on your peak water flow, you may fill a 1-gallon bucket with water and time how long it takes your shower head and other fixtures such as kitchen faucets, bathroom faucets, and other fixtures to fill it up.

Use this formula instead of filling an entire gallon per outlet if you don’t want to squander a full gallon each outlet: Flow rate = 15 / Number of seconds necessary to fill a 14-bucket container

Step 2: Temperature Rise

The following step is to calculate the temperature rise that is necessary. All you have to do is subtract the temperature of your input water from the temperature of the desired output water in this situation. Output water temperature minus feed water temperature equals required temp raise. What is the best way to determine the temperature of your feed water? There are two alternatives available to you:

  1. Measure using a thermometer
  2. Use our fantastic groundwater temperature map for the United States

Please keep in mind that these are approximations of typical temperatures. The actual temperature varies depending on the season and weather. As you can see, the location of your home in relation to the average groundwater temperature has a significant impact on the temperature of the water. The temperature will naturally be greater in warmer areas in the south, reaching up to 77 degrees Fahrenheit in southern Florida. On the other hand, groundwater temperatures may drop to as low as 37 degrees Fahrenheit in Alaska, sections of North Dakota, Minnesota, Wisconsin, and other northern states with colder climates.

Assume you reside in Michigan, where the average feed water temperature is 40 degrees Fahrenheit.

105 to 115 degrees Fahrenheit output water temperature is considered ideal for everyday household use, so 110 degrees Fahrenheit minus 40 degrees Fahrenheit equals 70 degrees Fahrenheit.

Showering at 105 degrees Fahrenheit is regarded to be the top end of the temperature range that is most pleasant. You want something about 110 degrees Fahrenheit at your kitchen sink.

Step 3: Putting It All Together

Okay, you know how much hot water you’ll need during peak hours, as well as the temperature spike that will be necessary. In order to complete this process, you must go out and seek for a tankless water heater that satisfies all of the standards. Almost all manufacturers include sizing charts with their goods, which state maximum flow rates for a specific temperature rise or vice versa, depending on the product. Some manufacturers additionally provide flow rates for various input and output water temperatures.

Keep in mind, though, that manufacturers tend to advertise their products by highlighting the best-case situations, so you should treat the information with caution.

Are you ready to take the next step?


Do you prefer video? Take a look at this:

Size Chart

If you look at a few size charts, you will immediately discover that the use of gas or electricity makes a significant difference. In general, tankless gas water heaters are more powerful than electric water heaters, which means that they can produce more gallons per minute (gpm) at the same temperature increase. As an illustration: If you want a temperature rise of 70 degrees Fahrenheit, a big tankless gas water heater can provide 5.0 – 5.5 gallons per minute at that temperature. The biggest electric unit (36 kW) produces a maximum flow rate of little more than 3.0 gpm.

Example Size Chart

For your convenience, the following is an illustration of a tankless water heater size chart: 6.6 gpm at a temperature rise of 50 degrees Fahrenheit and 4.8 gpm with a temperature rise of 70 degrees Fahrenheit for this specific heater.

Popular Tankless Water Heaters

Here are some real-world examples of tankless water heaters that are widely used: (Tip for mobile users: Swipe to scroll.)

Model Fuel Type, Power Temp Rise Max GPM of Bathrooms
Rinnai V75iN Natural gas, 180,000 BTU 70 °F 4.3 gpm 1 – 2 bathrooms
50 °F 6.0 gpm 2 – 3 bathrooms
Rinnai RU199iP Propane, 199,000 BTU 70 °F 5.5 gpm 2 bathrooms
50 °F 7.6 gpm 3 bathrooms
Stiebel Eltron Tempra 36 Plus Electric, 36 kW 70 °F 3.5 gpm 1 bathroom
50 °F 4.75 gpm 1 – 2 bathrooms
Rheem RTEX-18 Electric, 18 kW 65 °F 2.0 gpm 1 bathroom
55 °F 2.0 gpm 1 bathroom
EcoSmart ECO 11 Electric, 11 kW 68 °F 1.1 gpm 1 bathroom
48 °F 1.56 gpm 1 bathroom

Frequently Asked Questions (FAQ)

Which size tankless water heater do I need to provide hot water for a family of five? It’s virtually hard to determine from the information provided. It is dependent on the number of bathrooms and water-using appliances you have, as well as the temperature rise necessary. Follow the steps 1 through 3 in our sizing guide, or use our tankless water heater sizing calculator, and you will receive a satisfactory solution to your concern – guaranteed. I need to replace my 50-gallon water heater, but what size tankless water heater do I need to do so?

  1. It is dependent on the number of bathrooms and water-using appliances you have, as well as the temperature rise necessary.
  2. 3.
  3. The number of tankless water heaters you require is determined by your peak hot water consumption and the temperature rise that is necessary (check sizing guide above).
  4. Electric tankless water heaters are ideal for households with fewer people or for tiny flats.
  5. Furthermore, even though it is more expensive up front, putting two smaller units in series can often be more cost effective than installing a single large unit at a single location.
  6. Thank you for your time!
  7. She is the head of content production and has completely immersed herself in the home water treatment sector, resulting in her becoming an expert in the field herself.

Gene enjoys reading books on philosophy and social topics, producing music, and going on hikes when he is not at BOS. Find out more about.

What Size Tankless Water Heater Should You Buy: Sizing For Your Family Needs

Tankless water heaters are the way of the future when it comes to hot water usage in residential and commercial buildings. Tankless water heaters are characterized by their small form and ability to provide hot water on demand. On the other hand, on-demand does not always imply continuous service. Flow rate is the amount of water that may be produced in one minute by a tankless water heater (both electric and natural gas). If the demand for hot water in your household exceeds the capacity of your water heater, you may experience a lack of hot water.

  1. Gallons per minute (GPM) is the unit of measurement for tankless water heaters (GPM).
  2. During peak hours, the average family consumes around 6.5 GPM.
  3. A tankless water heater with a capacity of 4 GPM can deliver enough water for one shower and one appliance to run at the same time.
  4. When a tankless water heater is undersized, it will not be able to provide enough hot water to fulfill the needs of your household.
  5. A tankless water heater with a flow rate of 4GPM will be unable to heat enough water to fulfill this demand for hot water.
  6. A lar ge tankless water heater has greater installation and operating expenses than a traditional tank water heater.
  7. For example, a 4 GPM tankless water heater will cost between $400 and $700 plus installation, and an 8+ GPM tankless water heater would cost between $900 and $2000 plus installation.
  8. Hot water on demand is defined as having continuous hot water output for the duration of time that the hot water tap is open.
  9. If you have too many hot water faucets open at the same time, the water will become lukewarm if the tankless water heater’s output capacity is surpassed.
  10. Installation costs are in addition to the cost of the tankless water heater.

We’ve put together this complete shopping guide to assist you in making the best decision for your family when it comes to tankless water heaters. The following section will lead you through the many various considerations you should make while shopping for tankless water heaters.

Gallons of Hot Water You Need for a Tankless Water Heater

The first thing you’ll need to think about is how much hot water you’ll require from your tankless heater. In most cases, we refer to this as “sizing,” although you will not be using dimensional measures to determine the appropriate size for your tankless water heater. Instead, you’ll need to pay attention to two separate metrics: the flow rate and the temperature rise (or decrease).

See also:  How To Replace Refrigerator Water Filter

Tankless Water Heaters GPM Flow Rate Explained

It is the time of day when you use the most hot water at one time that is referred to as peak demand. Tankless water heater size is determined by the flow rate, which is measured in gallons per minute (GPM). The majority of tankless water heaters are capable of producing between 2 and 10 gallons of water per minute on average. During peak demand periods, it will be necessary to total up all of the flow rates of the fixtures and appliances in your home that may be using hot water at the same time in order to establish your water flow rate need during those times.

However, the following list of normal flow rates for popular fixtures and appliancesshould assist you in making an estimate:

Fixture or Appliance Usage: AverageGallons per Minute(GPM) Used
Bathtub 4 GPM
Rainhead shower head Up to 5 GPM
Standard shower head 2.5 GPM
Standard dishwasher 2.5 GPM
High-efficiency dishwasher 0.5 – 1.5 GPM
Standard clothes washer 2.5 GPM
High-efficiency clothes washer 1.0 GPM
Sink faucet 1.5 GPM

Using the expected GPM production as a guideline. There are some fixtures and appliances that have a larger output than others. Using a normal shower head (2.5 GPM) consumes approximately half the amount of hot water as using a rainhead shower head, as seen in the following table (5.0 GPM). The kitchen faucet consumes approximately the same quantity of hot water as other sink faucets, regardless of where they are installed. It is uncommon for many sink faucets to function in tandem for extended periods of time.

What is a Tankless Water Heater’s Temperature Rise

Once you’ve determined the amount of water flow your tankless unit will require during peak demand periods, you’ll need to figure out how much “temperature increase” it will require. The temperature rise refers to the amount of degrees your hot water heater will need to raise the incoming average groundwater temperature in order to supply enough hot water for your household. For example, suppose the temperature of the entering cold water is 52 degrees Fahrenheit. If you want hot water that is 110°F, you’ll need a temperature raise of 58°F.

Putting Flow Rate and Temperature Rise Together

The flow rate and temperature rise of a water heater are both measured in the ratings. If you want to appropriately size your water heater, you must take into consideration both the flow rate necessary and the rise in temperature required.

Ground Water Temperature Expected Gallons per Minute Output
37 F 3.9 GPM
42 F 4.7 GPM
47 F 5.5 GPM
52 F 6.3 GPM
57 F 7.1 GPM
62 F 7.8 GPM
67 F 8.6 GPM
72 F 9.4 GPM

Another example is based on a tankless water heater with a 9.5 GPM output and a temperature rise of 35 degrees. Consider the following scenario: you have four gadgets that may all demand hot water at the same time:

  • 1 GPM / 110° for the hot water faucet
  • 1 GPM / 110° for the dishwasher
  • 2 GPM / 120° for the washing machine.

1 GPM / 110° for the hot water faucet; 1 GPM / 110° for the dishwasher; 2 GPM / 120° for the washing machine

Are Tankless Water Heaters Worth the Cost

When it comes to purchasing a tankless water heater, many individuals question if the investment is worthwhile. Tankless water heaters, in contrast to tank water heaters, are a good investment because of their longer lifespan, lower maintenance requirements, and ability to provide continuous hot water on demand.

  • Tankless water heaters have a lifespan of up to 20 years, whereas tank water heaters have a lifespan of as short as eight years. Due to the fact that tankless water heaters do not have a tank that might malfunction and leak, the danger of water damage is reduced
  • Tankless water heaters do not require any maintenance because they do not have a tank. Tank models require yearly flushing maintenance to maintain the tank’s integrity, which is commonly neglected, resulting in a shorter tank’s lifespan as a result. Tankless water heaters have the ability to produce hot water on demand when needed. A 50-gallon tank water heater can get depleted in as short as 30 minutes of continuous usage and requires a minimum of one hour of recovery time
  • However, this is not the case.

Electric vs. Gas Tankless Water Heaters

Once you’ve calculated the appropriate flow rate and temperature rise, you’ll need to choose between electric and natural gas tankless water heaters for your application.

Electric Tankless Water Heaters Cost Less to Buy and Install

It is one of the key advantages of purchasing an electric tankless water heater over purchasing a gas-powered tankless water heater because they are less expensive. Tankless electric water heaters for the entire house normally cost between $500 and $700, but whole-house tankless gas water heaters cost between $1,000 and $1,200 or more. Aside from that, the cost of installing an electric tankless heater is much lower than the cost of constructing a gas-powered tankless heater. Following your confirmation that your home’s wiring is suitable with the type you’ve chosen, installing the electric heater should be a reasonably simple and quick task to complete.

In order to work, some electric water heaters require up to four 220v breakers, which dramatically increases the expenses of installation and operating.

For starters, you’ll need to build a ventilation system to safely exhaust any fumes that your water heater may produce.

Electric Tankless Water Heaters are Easier to Maintain

Because electric units are simpler devices than gas-powered units, they are less expensive than gas-powered units. Because there are fewer components within, there are fewer moving parts that might break down and necessitate a costly repair job. The cost of fixing a propane-powered device is often cheaper than it would be if the equipment were fueled by gas. The majority of tankless water heaters will beep to alert you when tankless water heater maintenance is required.

Electric Tankless Water Heaters are More Energy-Efficient

The most efficient gas-powered tankless heaters have an energy efficiency of around 85%. When compared to most electric vehicles, which have an average energy efficiency of 98 percent, an 85 percent efficiency rating is considered inadequate. So even though natural gas is less expensive than electricity in your location, choosing an electric unit may result in you saving money on energy expenditures in the long run. Electric heaters are more ecologically friendly than other types of heaters.

The production of electricity is already less detrimental to the environment than the combustion of natural gas, and the greater energy efficiency of electric vehicles further adds to the disparity between the two modes of transportation.

Electric Tankless Water Heaters Can Require Significant Electrical Upgrades

The most important drawback of electric tankless water heaters is the difficulty in installing them on the premises. A separate 240v breaker is required for each burner on an electric tankless water heater. For large GPM models to work properly, three or four 220v breakers must be installed, which means you may need to install an electrical subpanel or make other electrical changes. If you choose an electric tankless water heater with a high output, you may need to build a new subpanel to supply electricity to the water heater.

You’ll need eight breaker slots to accommodate four 240v breakers.

According to industry standards, one heating element is required for every 2GPM of water production in an electric tankless unit.

Gas Tankless Water Heaters Produce More Hot Water

A variety of natural gas and propane types are available for tankless gas water heaters. Some purchasers choose a tankless gas water heater over an electric water heater because gas versions can handle higher flow rates than electric water heaters. The best electric units have a maximum flow rate of 8GPM, but some gas ones have a flow rate that is substantially greater. In the case of a big home or an industrial application, you may be forced to choose a tankless heater that is fueled by natural gas.

The majority of on-demand tankless water heaters are capable of operating at a variety of intake temperatures.

In an electric model, the same rise in temperature of 70 degrees Fahrenheit (70 degrees Fahrenheit) will result in around 2 gallons of hot water per minute in hot water, which is a considerable drop in production.

Natural Gas is Cheaper than Electricity in Some Areas

Given the price disparity between natural gas and electricity, depending on where you reside, a gas-powered device may be more appealing than an electric model. However, even if natural gas is much less expensive than electricity, the gain in efficiency associated with electric models, as well as the anticipated increase in natural gas pricing, may still make turning electric the most cost-effective alternative.

Indoor or Outdoor Installation Options

In contrast to a standard tank-style water heater, which must be installed indoors, tankless water heaters may be installed either indoors or outdoors. Installation of your tankless water heater outside has the primary advantage of eliminating the need to install a venting system inside. Because electric units do not require ventilation, placing them outside provides no meaningful advantage in terms of energy savings. In warmer areas, it is possible to put a tank-style water heater outside; however, you will still need to construct an enclosure around it.

The majority of outdoor tankless water heaters include built-in freeze protection that can withstand temperatures as low as -30 degrees Fahrenheit.


Although you may not give it much thought, you use your hot water heater on a regular basis. Providing hot water whenever you need it is the one item that helps keep things clean and operating smoothly in any household! The question is, how do you choose the appropriate size tankless water heater for your household? Take into consideration the appliances that will be used at the same time. When two persons living in an average-sized home need to shower every day at different times of the day (for example, because they work opposing shifts), a 2 GPM model would be appropriate.

DISCLAIMER: The information provided on is not intended to be professional guidance.

It is owned and operated by Hubert Miles who is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by placing advertisements and links on their websites that direct traffic to (hereinafter referred to as “” or “ Associates Program”).

Hubert Miles receives a commission for recommending visitors and commerce to these businesses.

Leave a Reply

Your email address will not be published.