Water Heater Size Calculator
Size a tank water heater by peak-hour hot water demand (first-hour rating), or a tankless heater by flow rate and temperature rise.
How to use
- 1Enter how many showers, baths and appliances run in your busiest hour.
- 2For tankless, enter the flow of fixtures used at once and the incoming water temperature.
How it's calculated
Peak-hour demand ≈ showers × 20 gal + baths × 25 + dishwasher 6 + clothes washer 7 + kitchen sink 4 (DOE estimates). Tankless: flow (gpm) and temperature rise determine kW/BTU: BTU/h = gpm × 500 × ΔT°F.
Frequently asked questions
What is the first-hour rating?
How much hot water a tank heater can deliver in an hour starting full — shown on the EnergyGuide label. Choose one at or above your peak demand.
Why does incoming water temperature matter for tankless?
The heater must raise water from its incoming temperature to about 120 °F. Cold climates need a larger rise and more power.
Is a bigger tank always better?
No — you pay to keep unused water hot. Size for the busiest hour, not the busiest day.