Home Energy Calculators

Tankless water heater GPM.

A tankless water heater is sized by the hot water flow it must deliver at your temperature rise: Btu/hr = 500 x gpm x rise in degrees F. Enter the fixtures that may run at once and your inlet and target temperatures to get the flow, heat output and, for electric, the amps. Runs in your browser; nothing is sent or stored.

Your numbers

Units

Switching resets the example values to the matching unit.

Showers running at once

Flow per shower (gpm or L/min)

EPA: standard showerheads use 2.5 gpm; WaterSense labelled 2.0 gpm or less.

Faucets or appliances running at once

Flow per faucet or appliance (gpm or L/min)

Read it from the fixture or its manual. 1.5 is an example.

Another fixture (optional): number

Another fixture (optional): flow (gpm or L/min)

Temperature at the fixtures (°F or °C)

The mixed temperature you want at the tap. Example value.

Cold water inlet temperature (°F or °C)

Use the coldest month where you live. Example value.

Heater output setting (°F or °C)

The temperature the heater delivers. Example value.

Heater efficiency

Percent, from the spec sheet. 95 is an example.

Supply voltage for an electric unit (optional)

Leave blank for gas.

A unit you are considering: input rating (Btu/hr, optional)

For an electric unit, kW x 3,412. 199,000 is an example.

Result

Fill the form and press Calculate.

Worked example

Two showers at 2.5 gpm and one faucet at 1.5 gpm running together draw 6.5 gpm at 105°F. With 50°F inlet water and a 120°F setting the rise is 70°F and the hot share is 55 / 70 = 79%, so the heater must deliver 5.11 gpm. Heat needed is 500 x 5.107 x 70 = 178,750 Btu/hr, or 188,158 Btu/hr of input at 95% efficiency, which is 52.4 kW or about 218 amps at 240 V. A gas unit with a 199,000 Btu/hr input at 95% delivers up to 5.40 gpm at that rise, enough. Example inputs: replace them with your own.

How it works

Btu/hr = 500 x gallons per minute x temperature rise (°F). One Btu heats one pound of water by 1°F (US EIA). A US gallon of water weighs about 8.345 pounds, and 8.345 x 60 minutes = 500.7, which is rounded to the familiar 500. Temperature rise is the heater setting minus the cold inlet temperature. Because tap water is a mix of hot and cold, the hot flow from the heater is the fixture flow x (use temperature - inlet) / (setting - inlet). Kilowatts are Btu/hr divided by 3,412 (EIA), and amps are watts divided by volts.

Not covered. A unit of fixed capacity can heat fewer gallons per minute as the rise grows, so use the maker's flow-versus-rise chart for the model you pick. Gas units need adequate venting and gas supply, and electric ones a suitably sized circuit, per the maker and a qualified installer. This is a planning estimate, not installation advice.

Sources and basis (pages opened 2026-10-08): US EIA, What is a British thermal unit (Btu)?; US EPA WaterSense, Showerheads.

This is a planning estimate and not professional plumbing, electrical or financial advice. Rates, fixtures and equipment vary, so confirm important decisions with your utility, the manufacturer or a licensed professional.

FAQ

How do I size a tankless water heater?

Add up the flow of the fixtures that may run at once, work out the temperature rise from your coldest inlet water to the heater setting, then use Btu/hr = 500 x gpm x rise to find the heat output you need.

What does the 500 mean in the tankless formula?

A US gallon of water weighs about 8.345 pounds and a Btu heats one pound by 1 degree F. Multiplying 8.345 by 60 minutes gives about 500 Btu per hour for each gpm and each degree F of rise.

Why does a tankless heater deliver less flow in winter?

Colder inlet water means a bigger temperature rise, and a fixed-capacity unit can heat fewer gallons per minute through a bigger rise. Size for your coldest month.

Is a bigger Btu rating always better?

Not necessarily. Match the unit's flow at your rise to your simultaneous demand; very large units cost more, and electric ones may need a much bigger electrical supply.

Can I use litres per minute and degrees Celsius?

Yes. Switch the units setting. The calculator converts to gpm and degrees F internally and shows both.