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

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Tankless water heaters provide endless hot water on demand. Furthermore, they can reduce your utility bills by more than 30 percent due to the elimination of standby energy losses, they are about the size of a suitcase and therefore space-saving, and they last up to 20+ years which is almost twice as long as what you get out of a regular tank-based unit.

However, tankless water heaters are limited by how much water they can heat at any given time. Choose too small of a system and it won’t have the capacity to provide enough hot water to meet your household needs. Go too big and you will overpay. This is why proper sizing is key – before making a purchase.

This buying guide will walk you through the process of sizing a tankless water heater and also shed light on other aspects that you have to consider when shopping.


How to Size a Tankless Water Heater

Simply put, tankless a.k.a. on-demand water heaters are rated by their maximum output water flow rate at a specific temperature rise.

So, in order to size a tankless water heater you have to consider 2 things:

  1. How many gpm (gallons per minute) of hot water you need at times of peak consumption. Remember, tankless water heaters don’t store any water but heat it as it flows.
  2. The required temperature rise which is determined by feed water temperature and output water temp settings.

What if you undersize?

Bad idea! If the required hot water demand surpasses the possible maximum, even if only temporary, most water heaters will throttle the flow automatically. This means less hot water at every outlet and therefore a temperature and/or pressure drop.

In addition, a tankless water heater that runs at full speed all the time is more likely to break down prematurely.

What if you go too big?

Oversizing is not that big of a deal. The only issue is unnecessary upfront cost.

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

Let’s start by figuring out your peak hot water demand. You can use the chart below for this. It lists the standard flow rates of different types of water outlets sold in the U.S. Of course, you could also look up your specific fixtures etc. online or check their product manuals.

Simply determine which devices you want to be able to run simultaneously and how many of them. Then add up their flow rates.

Kitchen Faucet

For example, if you need 1 shower and 1 kitchen faucet running at the same time your required maximum water flow is:

2.5 gpm + 2.2 gpm = 4.7 gpm

For WaterSense-certified products, the required flow rate is:

2.0 gpm + 1.5 gpm = 3.5 gpm

Pretty simple! But before you get started, here are a few more tips:

  • By planning for maximum demand at peak hours like in the morning you can ensure that you always have hot water at the ready no matter what. Above that, a tankless water heater that does not have to run at full capacity all the time is likely to last longer. At the same time, a little planning will allow you to reduce your peak demand significantly. This includes taking turns when showering, letting the dishwasher do its thing while everyone is out of the house, and preparing your food before or after the showers are finished.
  • Obviously, you have to take into account not only the number of bathrooms in your home, but also how many people are living under one roof. 2 people can only use 2 showers at the same time, even in a 5-bathroom mansion.
  • Ask yourself: Will the water heater serve your entire home or just parts of it?
  • The flow rates listed below indicate total water output, so hot water PLUS cold water. In other words, showering with a 2.5 gpm shower head does not mean that all of the water will be hot. It is more likely that you mix in some cold to get the desired temperature. Thus, the actual hot water demand is somewhat lower.
  • To reduce flow rates you could install low-flow aerators or fixtures.
Water OutletStandard Flow Rate
Hand washing sink0.5 – 1.5 gpm
Shower head2.5 (2.0*) gpm
Bathroom faucet2.2 (1.5*) gpm
Bathtub faucet3.0 – 4.0 gpm
Kitchen faucet2.2 gpm
Washing machine23+ gallons per load, gpm hard to determine
Dishwasher6 gallons per load, gpm hard to determine

*WaterSense certified products

Please note: Older fixtures are likely to have higher flow rates.

As you can see, we did not include flow rates for washing machines and dishwashers. This is because we found it very difficult to get reliable data on this. Some sources say 2 to 3 gpm, others 1.5.

What you could do is run each appliance separately and monitor your water meter and measure time. This will give you a rough idea of how much flow is needed.

Speaking of measuring, for a more scientific approach to determining your peak water flow you can take a 1-gallon bucket and track how long it takes your shower head, kitchen faucet, bathroom taps, etc. to fill it. Then use the following formula to calculate the flow rate for each outlet:

Flow rate = 60 / Seconds required to fill bucket

If you don’t want to waste a full gallon per outlet, only fill a quart and use this formula instead:

Flow rate = 15 / Seconds required to fill ¼ bucket

Step 2: Temperature Rise

The next step is to determine your required temperature rise. Here all you have to do is subtract the temperature of your feed water from the desired output water temp.

Required temp rise = Output water temp – Feed water temp

How can you find out what temperature your feed water has? You have two options:

  1. Measure with a thermometer
  2. Use our awesome U.S. average groundwater temperature map

USA average groundwater temperature map colored

Please note: These are average temperature estimates. Real temperatures vary with season and weather.

As you can see, where you live in the country plays a huge role as far as average groundwater temperature is concerned. In warmer climates in the south, the temperature will naturally be higher, up to 77 °F in southern Florida. Whereas in Alaska, parts of North Dakota, Minnesota, Wisconsin and other states in far northern regions the groundwater temp can be as low as 37 °F.

Now, this makes a big difference as to how much a tankless water heater has to work to bring water up to the desired temperature. Let’s say you live in Michigan with 40 °F average feed water temperature. And you aim at 110 °F output temp. The required temperature rise would be

110 °F – 40 °F = 70 °F

A water heater installed in a home in Texas with 70 °F inlet water temp has to warm up the water by as little as

110 °F – 70 °F = 40 °F

By the way, 105 to 115 °F output water temp is considered ideal for everyday household use. 105 °F is considered the upper end of the temperature range most comfortable for showering. At your kitchen sink you want something around 110 °F.

Step 3: Putting It All Together

Okay, you know your hot water demand at peak hours as well as the required temperature rise.

The final step is to go out and look for a tankless water heater that meets these requirements. Basically all manufacturers provide sizing charts with their products that specify maximum flow rates at a given temperature rise or vice versa. Some manufactures also list flow rates for different input/output water temps.

You want to choose a unit size that either meets or exceeds your peak hot water demand.

Be aware, though, that manufacturers like to promote their products using best-case scenarios, so take the information with a grain of salt. Some companies tend to exaggerate with what their systems can do…

Ready for the next step? Check our electric tankless water heater reviews to find a system that perfectly fits your needs!


Prefer video? Check this out:

Size Chart

Check out a few size charts and you will quickly notice that gas or electric makes a huge difference. Generally speaking, tankless gas water heaters are more powerful, meaning that they can produce higher gpm than electric units at the same temperature rise.

For example:

If your required temperature rise is 70 °F, a large tankless gas water heater can supply 5.0 – 5.5 gallons per minute. The largest electric unit (36 kW) maxes out at little more than 3.0 gpm.

Example Size Chart

For reference, this is what a tankless water heater sizing chart looks like:

water softener example sizing chart

This particular heater can provide 6.6 gpm at a 50 °F temperature rise and 4.8 gpm at 70 °F.

Popular Tankless Water Heaters

These are some real-life examples of popular tankless water heaters:

(Hint: Swipe to Scroll)

ModelFuel Type, PowerTemp RiseMax GPM# of Bathrooms
Rinnai V75iNNatural gas, 180,000 BTU70 °F4.3 gpm1 – 2 bathrooms
50 °F6.0 gpm2 – 3 bathrooms
Rinnai RU199iPPropane, 199,000 BTU70 °F5.5 gpm2 bathrooms
50 °F7.6 gpm3 bathrooms
Stiebel Eltron Tempra 36 PlusElectric, 36 kW70 °F3.5 gpm1 bathroom
50 °F4.75 gpm1 – 2 bathrooms
Rheem RTEX-18Electric, 18 kW65 °F2.0 gpm1 bathroom
55 °F2.0 gpm1 bathroom
EcoSmart ECO 11Electric, 11 kW68 °F1.1 gpm1 bathroom
48 °F1.56 gpm1 bathroom

Buying Guide: How to Choose a Tankless Water Heater

We’ve already covered sizing. But if you want to go tankless you have to take more into account than just that…

Types of Tankless Hot Water Heaters

Tankless water heaters can be categorized by their fuel source as well as application type. Let’s start with the former.

Fuel Type: Electric or Gas?

When you are upgrading your home from a tank water heater to a tankless system, sticking with the same fuel type can make sense as it may help to save money on installation. If you are building a new home, all that matters is availability and cost of the different fuel types.

There is electricity, natural gas – typically the least expensive option though prices vary by region – and propane which is more efficient than natural gas due to much higher BTU.

If you are fortunate enough to have the choice, make sure to compare the cost for each. Natural gas is delivered via pipeline and therefore not available everywhere. But if it is, it is usually the preferred type. Propane has to be purchased independently.

What Is BTU?

1 BTU = the amount of energy required to raise the temperature of 1 lb of water by 1 °F.

The higher a heater’s BTU rating, the more hot water it can provide per minute.

Like we said, in general gas-fired water heaters have more power than units running on electricity, so they can heat the water more quickly. In fact, electric tankless water heaters are only partially suited for whole house use. Even the biggest models can barely provide enough water for 1.5-bathroom homes, unless you live in the south.

Still, electric tankless water heaters have 2 major advantages:

  • They are much more affordable than gas units.
  • And they are easier to install, provided that the setup does not require rewiring of your home (more on this below). This saves you even more money and time.

Gas water heaters perform better from an energy-savings point of view which will lower operation cost.

Whole House or Point-of-Use?

You can choose between a whole house tankless water heater and one or multiple point-of-use systems that work independently from one another. (Of course, you could also install two whole house units to work in tandem in case that’s needed, which is rare.)

The benefit of point-of-use is that you can install them directly where the hot water is needed, at least in case of an electric unit, allowing for instant hot water. Since no venting is required (learn more here), they can even be installed under your kitchen sink or in a closet.

Where a lot of hot water is being used – think hot tub – or in a remote area of your home that could do with an extra boost, a point-of-use water heater may also be perfect to supplement a central whole house system. Several units can be used to meet the demand of larger families.

All in all, once you have your required flow rate and temperature rise you will know whether whole house or point-of-use will work best for you. Again, tankless gas water heaters are more common with whole house use, electric are more geared towards point-of-use, except when used in warmer climates and/or for small places.

Energy Factor (EF)

A system’s Energy Factor rates its energy conversion efficiency. A higher factor equals higher efficiency. This becomes important if you are concerned about energy savings which directly translate to lower operating cost.

The formula used to calculate a water heater’s EF is:

EF = Energy converted into water heat / Total amount of energy used

energy label

For modern electric tankless water heaters, the U.S. minimum efficiency standard is 0.93, although most systems easily achieve 98 percent and higher. This is why the Energy Factor only plays a minor role here.

For modern tankless gas water heaters, ratings range from 0.82 which is the current U.S. minimum efficiency standard to 0.98 for highly efficient condensing tankless gas water heaters. These systems use a second heat exchanger to capture excess exhaust heat to pre-heat the incoming water.

Source: https://www.nist.gov/system/files/documents/iaao/Healy.pdf

FYI: The fact that electric tankless water heaters are more energy efficient than gas units does not imply that they are cheaper to operate. In fact, the opposite is true because gas is the less expensive fuel source at this time.

Pilot Lights (Tankless Gas Water Heaters Only)

Some tankless gas water heaters use a standing pilot light that burns constantly and thus requires fuel to maintain a flame even when no hot water is being used. This is a waste of energy, obviously.

The only advantage that heaters with standing pilot lights have is that they are more affordable.

The better alternative is models making use of intermittent ignition that only sparks the burner when water is flowing.


When talking about tankless water heater installation, factors such as your home’s plumbing configuration, size of the gas supply, and wiring and electrical service come into play.

Generally speaking, switching to tankless is expensive (if you are building a new home you definitely want to go tankless from the beginning). And as you will see, in most cases a gas system is more costly to install than an electric unit. The good news is that your tankless water heater after that will be much cheaper to set up as no retrofitting will be involved.

Also, hiring a licensed contractor to do the setup for you is recommended – some manufacturers will even void the product warranty if a heater was not installed by a professional.

If you are a passionate DIYer and insist on doing the installation yourself, remember that there are building codes to comply with and that you will likely have to pull a permit.

water softener installation


The primary concern when installing an electric tankless water heater is amperage draw: Does your home have sufficient electrical service to accommodate the system?

For example, with the Stiebel Eltron Tempra 36 Plus which we’ve mentioned before in this guide the minimum recommended main service is 300A, plus it requires 3 separate dedicated 50A double pole breakers. To put this into perspective, the average home has a total capacity of 200A.

And even if you choose the smaller Tempra 24 Plus, 2x 50A double pole breakers and 150A main service is still required.

What this means is that you might have to upgrade your service panel with bigger wires and bigger breakers (you might be able to make these changes yourself in order to save some money). Or you have to run an entirely new panel which is a job for a trained electrician and can easily cost $1,000 USD and more.

At least hosting a point-of-use electric tankless water heater should not be a problem in terms of amperage requirements as long as your breaker panel isn’t full already.

On a side note: Product descriptions should specify amperage requirements. If you are unfamiliar with any of these numbers better consult an electrician before you spend your money.

Want more information on electric tankless water heater installation?


What do you have to consider when it comes to installing a tankless gas water heater? One word: Venting.

Venting allows you to get rid of the exhaust fumes that every gas water heater produces. And because tankless gas water heaters produce a lot of them, they need special, and most importantly larger, venting. So while electric tankless water heaters can be installed in tight spaces, more room is required with gas.

Also, if you are switching from tank-based to tankless reusing your old venting probably won’t work. This is because exhaust temperatures are much higher. Metal piping is almost always required.

It also makes a difference if a unit uses direct venting versus power venting. And non-condensing tankless gas water heaters require different venting than condensing units. But those are topics for another day. All you have to know for now is that venting done by a licensed professional will add to the installation cost.

Outdoor Tankless Water Heaters

An outdoor tankless water heater makes venting completely unnecessary. However, although there are measures for freeze protection, they are not for cold climates.

The second issue is gas supply in your home which needs to be large enough to power a tankless unit. A new (dedicated) gas pipe with a larger diameter might be needed, although sometimes it can be enough to increase supply pressure. We recommend you make an appointment with a certified plumber in your area to discuss your options.

Anyway, be prepared to spend at least several hundred and up to a couple thousand dollars for this.


Lastly, warranty length and coverage is also something you want to pay attention to. When you consider that tankless water heaters have a life expectancy of 20+ years, you should get at least several years of warranty coverage.

Frequently Asked Questions (FAQ)

1. What Size Tankless Water Heater Do I Need for a Family of 5?

This is almost impossible to tell. It depends on how many bathrooms and water using appliances you have, and the required temperature rise. Go through steps 1 to 3 in this sizing guide or use our tankless water heater sizing calculator and you will get an answer to your question – guaranteed!

2. What Size Tankless Water Heater Do I Need to Replace a 50-Gallon Water Heater?

Again, this is simply not enough information to give a reliable answer. It depends on how many bathrooms and water using appliances you have, and the required temperature rise. We recommend you go through steps 1 to 3 in this sizing guide or use our sizing calculator!

3. How Many Tankless Water Heaters Do You Need?

How many tankless water heaters you need depends on your peak hot water demand and required temperature rise (check sizing guide above).

In general, one tankless gas water heater should be enough to supply a mid-sized family/home. Electric tankless water heaters are perfect for smaller families and apartments.

In case of very high demand, consider installing 2 or more heaters, either at the point of use so they work separately from one another or in one central spot to work in tandem operating as a single unit.

Furthermore, although more expensive upfront, adding two smaller units in series can sometimes make more sense than installing one large one.

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If you have any thoughts or questions about how to size a tankless water heater please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald has been with BOS since the very beginning. She is head of content creation and has fully immersed herself into the home water treatment industry only to become an expert herself. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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