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What Size Tankless Water Heater Do I Need? – Detailed Answer

Fact checked by Stephen Conklin

what size tankless water heater do I need

Have you ever asked yourself, what size tankless water heater do I need? There is no need to fret because almost all of us know how it feels. The confusion stems from the absence of a tank that gives us a reference frame to make a decision when buying a tankless system.

However, there are proven methods to determine the ideal size of a tankless water heater for your home. While most techniques provide an estimate, it is still better than second-guessing your tankless water heater needs.

Table of Contents

What Do I Need to Determine the Heater Size Appropriate for My Home?


If you find yourself writing about how big of a tankless water heater I need, you are in luck because I will share some pointers. The first thing you must do is to determine three things: your maximum warm water requirements, the water heater’s water flow rate, and your expected temperature rise.

Hot Water Needs

Families have different warm water requirements. For example, some households only use warm water during bathing, while others love using warm water in their kitchen sink hot water faucets, washing machines, dishwasher bathroom faucets, and other water-using appliances and fixtures.

In general, the heater must supply enough hot water for everyone to use at the same time.

Flow Rate

Water appliances and fixtures have different flow rates expressed in gallons per minute (GPM). For example, showers typically have a flow rate of 2 GPM to 3 GPM, while washing machines will require slightly lower water consumption of 2 to 2.5 GPM.

On the other hand faucets have a flow rate of 1 to 2 GPM, while a dishwasher may use 1.5 to 2 gallons of hot water per minute. The flow rate describes continuous hot water use.

To size a tankless water heater, you must check your maximum hot water needs by considering the maximum flow rates of the different water appliances and fixtures.

You might want to determine the time of day when almost everyone in the family uses hot water. For example, one member may be showering, while another is brushing their teeth. Another family member might be washing the clothes or cleaning the dishes.

Temperature Rise

Most newbie owners forget to consider the given temperature rise or the temperature difference between the groundwater temperature and the temperature at water outlets.

For example, suppose the water entering the heater has a groundwater temperature of 50 degrees Fahrenheit, and the shower temperature is 100 degrees Fahrenheit. The required temperature rise is 50 degrees.

Why Do Tankless Systems Have Maximum GPM and Realistic GPM?

You might have noticed that tankless heaters have two GPM values on their spec sheets or labels. One is the maximum GPM, and another is the actual flow rate.

The maximum GPM reflects the water heater’s maximum water flow rate, meeting ideal conditions.

We know that it is impossible to achieve the maximum GPM in our households because of various factors. That is why manufacturers also include a realistic GPM, adjusted for the average groundwater temperature and accounting for the temperature rise.

For example, a tankless water heater size of 7 GPM might be accurate if one lives in warmer climates where the groundwater temperature is between 75 and 80 degrees Fahrenheit (North Texas, for example).

However, if you live in Minnesota, you can expect your incoming water temperature to be 35 to 37 degrees Fahrenheit. That is cold water for you.

Remember what I said about temperature rise or the difference between the input water temperature and the output temperature? Heating Minnesota water to a comfortable 110 degrees Fahrenheit requires the water heater to raise the temperature by 73 degrees.

If the heater retains its 7-GPM maximum flow rate, it cannot achieve the required temperature rise of 73 degrees. The system must reduce the water flow rate to ensure better contact between the water and the heating elements.

That is why households in 35-degree environments may only have a maximum flow rate of 3.5 GPM. This value is the heater’s realistic GPM.

Is There a Difference in the Sizing of an Electric vs. Gas Tankless Water Heater?


A tankless unit has a different sizing from a tank water heater. The latter size its water tank in total volume. On the other hand, a tankless hot water heater only requires determining how much hot water flows through the tankless heater. People love a tankless heaters for their space-saving properties.

Electric water heaters have a different tankless water heater sizing from gas units. You will see electric models and electric counterparts classified according to their wattage rating. Smaller electric tankless heaters have a low wattage rating.

In general, the higher the wattage rating of the tankless electric water heater, the more powerful the electric tankless water heaters in heating the water. Unfortunately, high-power electric units can also translate to higher operating costs, which you can find in your subsequent electric bills.

On the other hand, gas-powered units come with GPM values. We already discussed that the correct size of a tankless heater depends on your maximum warm water needs and the device’s realistic flow rate (considering temperature rise).

A tankless gas heater receives fuel from the natural gas line. Propane models heat the water faster than an electric tankless heater. You will also enjoy typical flow rates (near-ideal) in a warm climate using a tankless gas water heater.

Gas tankless water heaters are also more economical than electric tankless water heaters.

What Size Heater Should My Family Get?

Let us put together what we learned so far to determine the best possible size of tankless heater for your family. Here are the steps.

Step 1: Determine how much GPM or how much flow you need to use hot water simultaneously. For our example, let us work with the following information.

  • Shower: 2 GPM
  • Kitchen faucet: 1.5 GPM

Let us pretend that you have two bathrooms and two people shower at the same time, while another person uses the kitchen faucet.

We can compute the maximum warm water requirement as 5.5 GPM (two showers at 2 GPM each and one faucet at 1.5 GPM). It gives you a rough idea of what size tankless water heater you should get.

Step 2: Determine the temperature rise in your home. Measure the water temperature coming from the faucet to determine the incoming or groundwater temperature. Subtract this number from your desired temperature for the shower or the washing machine.

For example, suppose you want to bathe in the 110-degree water, and the inlet water temperature is 50 degrees Fahrenheit. Therefore, the temperature rise is 60 degrees Fahrenheit (110 minus 50).

You can also use a free online tankless water heater sizing calculator if you like.

Step 3: Pick the correct tankless heater size. In our example, the most appropriate tankless heater should deliver at least 5.5 GPM and elevate the temperature by at least 60 degrees Fahrenheit.

You can also check a tankless water heater size chart if you hate making computations. Most manufacturers have their water heater sizing chart. Hence, it would be a good idea to check with your favorite brand to determine the best possible heater size for your family.

For a family of two, most manufacturers recommend a 6 GPM to 8 GPM gas tankless water heater or a 10 kW to 18 kW electric tankless water heater.

If there are three people in your household, you can get a 7 GPM to 9 GPM gas heater or a 15 kW to 23 kW electric unit.

A four-member household might want to get a 20 kW to 28 kW electric heater or an 8 GPM to 10 GPM gas-powered unit.

On the other hand, a 9 GPM to 11 GPM gas water heater or 25 kW to 34 kW electric unit is sufficient for a five-member family.

Families with more than five people should get a gas water heater with at least 11 GPM or an electric unit with a minimum of 34 kW rated power.


What size tankless water heater do I need? You might ask. To sum it up, determining the size of a tankless heating appliance to install in your home requires understanding a few factors. These considerations include maximum warm water needs, the heater’s water flow rate, and the required temperature rise.

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