Tankless Water Heater Size Calculator

Add the flow of fixtures that run at the same time. Use your coldest-season inlet temperature — in northern states that can be 40 °F or lower.

Important: Gas sizing, venting and electrical supply must meet code and the manufacturer’s instructions. Large electric units may need multiple dedicated circuits.

Tapeline — Tankless Water Heater Size Calculator

Printed September 29, 2026

Flow rate4 gpm
Coldest incoming water temperature50 °F
Desired outlet temperature120 °F
FuelGas
Heater efficiency93 %
Supply voltage (electric)240 V

Results update as you type.

gpm
°F
°F
%
V

How it’s calculated

  • Output (BTU/h) = 500 × GPM × temperature rise
  • 500 ≈ 8.33 lb/gal × 60 min/h × 1 BTU/lb·°F
  • Input = output ÷ efficiency; amps = watts ÷ volts

Worked example

4 gpm with 50 °F inlet and 120 °F outlet, gas at 93% efficiency.

Inputs

Flow rate
4 gpm
Coldest incoming water temperature
50 °F
Desired outlet temperature
120 °F
Fuel
Gas
Heater efficiency
93 %
Supply voltage (electric)
240 V

Results

Required input rating
150,538 BTU/h
Heat delivered to water
140,000 BTU/h
Temperature rise
70 °F

Load this example into the calculator

Assumptions and limits

  • Efficiency is steady-state; use the manufacturer’s flow-vs-rise chart to confirm a model.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

What size tankless water heater do I need?

Two showers (about 2 gpm each) with a 70 °F rise need about 140,000 BTU/h delivered, roughly 150,000 BTU/h input for a 93% gas unit.

Why does winter matter?

Colder inlet water means a larger temperature rise, so the same heater delivers fewer gallons per minute.