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This water heater size calculator follows the U.S. Department of Energy method to size a new water heater. For a tank or heat pump water heater it estimates your peak-hour hot water demand, which is the first hour rating you need. For a tankless heater it adds up the flow rate and temperature rise.
How to Use the Water Heater Size Calculator
- Choose a tank (or heat pump) or a tankless water heater.
- For a tank, enter how many times each use happens in your busiest hour of the day.
- For tankless, enter how many showers and faucets run at the same time and your incoming water temperature.
Sizing a Tank Water Heater
Tank and heat pump water heaters are sized by their first hour rating (FHR): the gallons of hot water they can supply in an hour starting with a full tank. It is printed on the EnergyGuide label. Choose a model with an FHR at least equal to your peak-hour demand.
| Use | Average hot water per use |
|---|---|
| Shower | 20 gal |
| Shaving | 2 gal |
| Hand dishwashing or food prep | 3 gal |
| Automatic dishwasher | 7 gal |
| Top-loading clothes washer | 25 gal |
| Front-loading (H-axis) washer | 15 gal |
Example from the DOE: three showers, one shave and one hand dishwashing in the busiest hour total 60 + 2 + 3 = 65 gallons, so you need an FHR of at least 65 gallons.
Sizing a Tankless Water Heater
Flow rate = Sum of fixtures used at once (gpm); Temperature rise = Output − Incoming temperature
A showerhead uses about 2.5 gpm and a faucet about 0.75 gpm, so running both needs 3.25 gpm. If you do not know your incoming water temperature, assume 50°F; heating it to 120°F is a 70°F rise. Compare these numbers with the heater’s flow rate at that rise.
Tips
- Low-flow showerheads and faucets reduce the size you need.
- Electric tankless heaters need large electrical service; gas units deliver more flow.
- Set the water heater to 120°F to save energy and reduce scalding risk.
Sources
Frequently Asked Questions
What size water heater do I need for a family of 4?
It depends on peak-hour use. Three or four showers in the busiest hour usually means a first hour rating of about 65–85 gallons. Check the first hour rating on the EnergyGuide label rather than the tank size alone.
What is a first hour rating?
The amount of hot water a tank water heater can deliver in one hour starting with a full tank. Choose one that meets your peak-hour demand.
How do I size a tankless water heater?
Add the flow rates of fixtures used at the same time and find the temperature rise. Choose a unit that delivers that flow at that rise.
What temperature rise do I need?
Subtract your incoming water temperature from the output temperature. With 50°F incoming water and 120°F output, the rise is 70°F.
Read the guide: What Size Water Heater Do I Need? Tank and Tankless Sizing
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