Water Heater Size Calculator
Size a water heater by your busiest hour of hot water use. The calculator gives the first-hour rating to look for on a tank heater and the flow rate and temperature rise for a tankless one.
How to use the Water Heater Size Calculator
- Enter the number of people and your busiest-hour hot water uses.
- Enter the incoming water temperature and the hot water temperature you want.
- Compare the results with the ratings of the heaters you are considering.
How it works
Tank heaters are compared by their first-hour rating (FHR) — how many gallons of hot water they deliver in an hour starting full. Add up your peak-hour use (shower ≈ 10 gal, bath ≈ 20, dishwasher ≈ 6, clothes washer ≈ 7, sink use ≈ 3) and choose a heater with an FHR within 1–2 gallons of that figure.
Tankless heaters are sized by flow rate (gallons per minute of simultaneous use) at the required temperature rise — the difference between incoming water and the hot water setting. Colder groundwater in northern states means a bigger rise and a larger unit.
Typical tank size by household
| People | Tank (gas) | Tank (electric) |
|---|---|---|
| 1–2 | 30–40 gal | 30–40 gal |
| 2–3 | 40 gal | 40–50 gal |
| 3–4 | 40–50 gal | 50–65 gal |
| 5+ | 50–75 gal | 65–80 gal |
Frequently asked questions
What size water heater for a family of 4?
Usually a 50-gallon tank (or a tank with an FHR of 60+ gallons), or a tankless unit delivering about 6–8 GPM.
What is a first-hour rating?
The number of gallons of hot water a tank heater can supply in one hour starting with a full tank. It is printed on the EnergyGuide label.
What temperature should a water heater be set to?
120 °F (49 °C) is recommended for most homes — hot enough for washing while reducing scald risk and energy use.