What Size Tankless Water Heater Do I Need? (3 Step Guide)

Do you want to learn how to size a tankless water heater?

The two key things needed to size a tankless water heater is your household peak water usage (in gallons per minute)—as well as your local 'temperature rise'.

In this guide, I will go over...

  • Finding your peak flow rate in GPM
  • Determining your local temperature rise
  • And choosing the right tankless heater that matches your needs

Let's get started with this guide!

what size tankless water heater do i need

How To Size A Tankless Water Heater?

Tankless water heaters are highly efficient and space saving water heaters that are becoming more popular every year. But if you choose an undersized tankless unit, you may get lukewarm water. And if you oversize a tankless water heater, then you are just wasting money.

To correctly size tankless water heater, a homeowner first has to find their household peak flow rate.  The flow rate is the total amount of water a home uses during peak usage and it is measured in gallons per minute or GPM. 

Secondly, the temperature rise needs to be known. Temperature rise is the difference between the incoming cold water temperature and the desired output temperature which is usually 110 to 120 degrees Fahrenheit.  Once the flow rate and temperature rise is determined, you can consult with the capacity charts of tankless water heater manufacturers.

For more details, keep reading.

Read Also: What Are The Best Propane Tankless Water Heaters?

Step 1 - Determine Flow Rate

The first step in properly sizing a tankless water heater is to determine your home's flow rate.

Flow rate or 'peak demand' is how much water your household uses during peak usage which is usually in the morning for most people. Flow rate is calculated using the GPM metric which is gallons per minute of water usage.

To get the flow rate for your household, you will have to estimate the number of plumbing fixtures that will be used in your home during peak usage. For a three person household that has two full bathrooms, the peak usage may be two showers, a dishwasher, and a washing machine.

To find the flow rate in GPM of a plumbing fixture, you can look on the fixture itself for a stamp such as a toilet that has '2.3 gpm' marked on it. If there is no marking on the plumbing fixture, you will have to look up the manufacturer and model to find the flow rate. And if you don't know the model or want to just to a quick estimate, you can use average flow rates for that type of fixture.

You also may want to factor in that your total flow rate during peak usage will actually be less than your calculation because fixtures mix in cold and hot water—it isn't all hot water.

Here are some average flow rates for plumbing fixtures...

Read Also: How To Flush A Tankless Water Heater?


Average Flow Rate






2.5 GPM


5.5 GPM


1.5 GPM

Washing Machine


Step 2 - Find The Temperature Rise

Determining the temperature rise is a critical part of correctly sizing a tankless water heater.

The temperature rise is a simple formula that takes your desired water temperature which is usually in the 110 to 120 Fahrenheit range. And you need to subtract the average groundwater temperature (or the likely lowest temperature) in your local area.

The temperature for a given region usually stays within a narrow range year-round which is why the average temperature can be used.

Read Also: What Are The Best Condensing Tankless Water Heaters?

what size tankless water heater do i need

Source: EPA.Gov

Depending on your part of the country, your incoming water from the city or well may get to or approach freezing temperatures around 32F.

If you live in a tropical area such as south Florida, you may never get groundwater temperatures below around 65F.

Lets say your lowest expected wintertime temperature to be 45F. Well, if you want your tankless water heater to put out 120F, then your 'temperature rise' will be 75F which is 120F - 45F = 75F. In other words, your tankless water heater will have to raise the temperature of the incoming water supply by 75 degrees Fahrenheit in order to reach your target water temp.

Read Also: How To Adjust Temperature On Rinnai Tankless Water Heaters?

what size tankless water heater do i need (1)

Step 3 - Determine Capacity

Now that you are equipped with your total flow rate during peak usage as well as your required temperature rise—you will need to adjust the advertised 'max GPM' to see if it meets your requirements.

Tankless water heaters will usually advertise a flow rate at a particular temperature. For example, a Rinnai unit may advertise that it has a 5 GPM max flow rate @ 60F (incoming water temp). Well, if your groundwater temperature gets to near freezing temperatures (lets say 40F) then the GPM will be significantly less that the advertised 'max GPM'.

Unfortunately, there isn't a single formula for all tankless water heaters, you will have to consult with the manufacturer for their 'water flow curve'.

The picture below is from a Rinnai tankless water heater manual for three different tankless models (RL94, RLX94i, and RL75) and shows how the GPM drops as the temperature rise increases.

Read Also: What Are The Best Tankless Water Heaters In Cold Climates?

what size tankless water heater do i need

Final Thoughts

Finding a correctly sized tankless water heater isn't that hard to do, but you may want to consult a professional HVAC tech because it can be costly if you get it wrong.

The basics of sizing a tankless water heater is to first determine your household flow rate in GPMs by adding up plumbing fixtures you expect to use at the same time during peak usage (like in the morning). Then, you will have to calculate your temperature rise by subtracting out the incoming water temperature (average groundwater temp in your area) from your desired output temperature in the 110F to 120F range.

Equipped with these two numbers, you can shop around for the right sized tankless water heater.  You may also need to consult with the manufacturer flow rate charts to adjust the advertised max GPM for a specific model.

Read Also: What Are The Best Outdoor Tankless Water Heaters?

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