WaterHeaterSize Tip

Water heater BTU calculator

Heating water takes 1 BTU per pound per degree Fahrenheit, and a gallon weighs 8.33 lb, so one gallon per minute heated by 1°F takes 8.33 × 60 ≈ 500 BTU per hour. WaterHeaterSize multiplies your flow by 500 and by the temperature rise to give the heat output, then divides by burner efficiency for the gas input rating, or by 3,412 for kilowatts.

Uses of hot water in the busiest hour of the day:

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    Use the Tankless tab: set the fixtures for your flow and the incoming and outgoing temperatures.

    How the water heater size calculator works

    Tank heaters are sized by their first-hour rating (FHR): how many gallons of hot water they deliver in one busy hour, starting with a full tank. WaterHeaterSize adds up your peak-hour demand with the US Department of Energy worksheet figures per use: shower 10 gal, bath 20, shaving 2, hand dishwashing or food prep 4, dishwasher 6 and clothes washer 7. Pick a heater whose FHR is at least that total. Tankless heaters are sized by flow and temperature rise instead: the flow of every fixture that may run at once (shower 2.0 GPM, bathroom faucet 1.0, kitchen faucet 1.5, tub 4.0, dishwasher 1.0, washer 2.0) times the rise from the incoming water to the setting. Heat needed = GPM × 500 × rise in °F, in BTU per hour; divide by efficiency for gas input, or by 3,412 for electric kW.

    Tankless size by flow at a 70°F rise (50°F water heated to 120°F)

    FlowHeat outputGas input at 95%ElectricAmps at 240 V
    1 GPM35,000 BTU/h36,842 BTU/h10.3 kW43 A
    2 GPM70,000 BTU/h73,684 BTU/h20.5 kW85 A
    3 GPM105,000 BTU/h110,526 BTU/h30.8 kW128 A
    4 GPM140,000 BTU/h147,368 BTU/h41 kW171 A
    5 GPM175,000 BTU/h184,211 BTU/h51.3 kW214 A
    6 GPM210,000 BTU/h221,053 BTU/h61.5 kW256 A

    Groundwater is colder in northern climates in winter (often 40°F or less), which raises the rise and the size you need. Low-flow fixtures reduce it. Gas units need correctly sized gas lines and venting, and electric units need a dedicated circuit; have a licensed installer confirm the final choice. Results are estimates for planning.

    Frequently asked questions

    How many BTU to heat water?

    BTU/h = GPM × 500 × temperature rise in °F. 4 GPM heated by 70°F needs 140,000 BTU/h of output.

    Why 500 in the formula?

    Water weighs about 8.33 lb per gallon, and 8.33 × 60 minutes ≈ 500.

    What is the difference between BTU input and output?

    Input is the gas burned; output is the heat that reaches the water. Output = input × efficiency.

    Is WaterHeaterSize free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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