What Size Tankless Water Heater Do I Need? All Your Questions Answered

Co-authored by Amber Crain

Last Updated: December 3, 2022

If you're in the market for a new tankless water heater, the size you choose needs to be able accommodate your household—but how do you figure out your specific needs? Sizing a tankless water heater is simple once you determine 2 things: your peak hot water usage and required temperature rise. In this article, we'll walk you through the easy process for calculating both of these numbers. Then, we'll show you how to put it all together so you can pick out the right tankless water heater for your home.

Things You Should Know

Determine which hot water devices you run simultaneously in your home and add the flow rates in gpms together to get your home's peak flow rate.
On average, the flow rates are: 1 gpm for a sink faucet, 3 gpm for a tub, 2.5 gpm for a shower, and 3 gpm for a dishwasher or washing machine.
Measure your faucet's cold water temperature with a thermometer. Subtract the cold water temperature from 110 °F (43 °C) to get the temperature rise.
Look for a tankless water heater that meets your home's needs for peak flow rate and temperature rise. Size charts are listed on all products.

1

Find the flow rate for all water sources in your home.

Include faucets, tubs, showers, dishwashers, and washing machines. Think about where you use hot water in your home so you can figure out how many devices you’re powering. Then, write down the flow rate in gallons-per-minute (gpm) for each of your devices. Use the list handy below to determine the typical flow rate by device:^{[1]
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- Sink faucet: 1 gpm
- Bathtub: 3 gpm
- Shower: 2.5 gpm
- Dishwasher: 3 gpm
- Washing machine: 3 gpm

2

Calculate your home's peak hot water usage.

Add up the flow rates to determine your tank’s minimum flow rate. Peak usage is the when you’re consuming the greatest amount of hot water. Think about when you tend to run multiple devices simultaneously and what those devices are. Then, just total up the flow rates for those devices to figure out the minimum capacity your tankless water heater needs to be.^{[2]
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- Let's say your peak usage happens after dinner when you’re running the dishwasher, washing a load of clothes, and taking a shower simultaneously:
  - 3 gpm + 3 gpm + 2.5 gpm = 8.5 gpm. Your tankless water heater needs a minimum 8.5 gpm flow rate. That way, your heater's minimum flow rate can easily support your peak usage.

3

Find the temperature of the cold water in your house.

Test the cold water temperature at a faucet with a thermometer. The easiest way to figure out your home's baseline cold water temperature is to simply turn on any cold water faucet in your home and let it run for a few minutes. Then, use a thermometer to take the temperature of the cold water.^{[3]
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- It's important to know the water temperature in your house, also known as the ground water temperature, because that's the temperature the water will be before the heater starts working. The lower the ground water temperature, the more power the heater will need to use get the water hot.

4

Subtract the water temperature from 110 °F (43 °C).

110 °F (43 °C) is the average household hot water temperature. To determine the required temperature rise for your tankless water heater, just subtract the cold water temperature you just measured from the average hot water temperature.^{[4]
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- Let's say your cold water temperature is 62 °F (17 °C): 110 - 62 = 48.
  - The required temperature rise for your tank is 48 °F (9 °C). The new water heater will need to heat your water 48 °F (9 °C) in order to reach 110 °F (43 °C).

5

Pick a heater that supports your home's peak flow rate.

Use the heater's sizing chart to confirm flow rate and temperature rise. All tankless water heater manufacturers provide sizing charts for their products that indicate maximum flow rates at given temperatures. Choose a tankless water heater that either meets or exceeds your peak hot water demand.^{[5]
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Official site for the U.S. Department of Energy, which provides resources related to energy safety, conservation, and efficiency
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- For example, if your peak flow rate is 8 gpm, buy a tank that’s 8 gpm or higher. 10 gpm would be a good choice.
- Try to stay reasonably close to your gpm; there's no benefit to vastly overshooting your needs. If your peak flow rate is 8 gpm, you don't need a heater that can support 15 gpm. The higher the flow rate, the more expensive the device will be.

6

Choose a heater that matches your home's fuel source.

Tankless water heaters are either gas-powered or electric. What energy source does your home use currently? You can also look at your old water heater—if it was electric, make sure your new one is electric, too. Matching the water heater to your home's energy source is the most cost-effective and easiest solution. Switching from gas to electric (or vice versa) is expensive and can only be done by a professional.
- Gas-powered heaters are usually a bit more powerful. If your minimum flow rate is on the high side (8 GPM or higher), gas may be better a better choice.
- If you go with an electric tankless water heater, you'll also need to know how much electricity is required to support your flow rate (measured in kW). Use this handy chart to figure out your kW needs.