WaterHeaterSize Tip

Tankless water heater size calculator

A tankless heater must heat the water as it flows, so its size depends on how many fixtures run at once and how much the water has to warm up. WaterHeaterSize adds the flow of each fixture and multiplies by the temperature rise: two showers and a bathroom faucet use 5 GPM, and heating 50°F water to 120°F takes 175,000 BTU/h of heat, a gas unit of about 184,000 BTU/h input at 95% efficiency.

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

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    An electric tankless heater for the same flow would need 51.3 kW, which is more than most homes can supply.

    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

    What size tankless water heater for 2 showers?

    Two 2 GPM showers plus a faucet is about 5 GPM; at a 70°F rise that needs about 184,000 BTU/h gas input, so a 199,000 BTU unit.

    How many GPM can a 199,000 BTU tankless heater supply?

    About 5.4 GPM at a 70°F rise with 95% efficiency, or about 8.4 GPM at a 45°F rise.

    What temperature rise should I use?

    Your hot water setting (often 120°F) minus the coldest incoming water temperature, usually in winter.

    Is WaterHeaterSize free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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