Tankless Water Heater Size Calculator (GPM, Temperature Rise, BTU & kW)
Size a tankless water heater from your peak hot-water flow and the rise from winter inlet temperature to your setpoint: required Btu/h, gas input, electric kW and amps.
Formula
Temperature rise = setpoint − inlet. Heat output (Btu/h) = gpm × 8.34 lb/gal × 60 min × rise. Gas input = output ÷ efficiency. Electric kW = output ÷ 3,412.14; amps = kW × 1,000 ÷ volts.
Worked example
Inputs: Peak hot-water flow (fixtures used at the same time) = 4.5 gpm; Coldest incoming water temperature (winter) = 52 °F; Hot water setpoint = 120 °F; Gas unit efficiency (spec sheet) = 82 %; Electric supply voltage = 240 V.
| Temperature rise °F | 68 |
|---|---|
| Heat output Btu/h | 153,122 |
| Gas input Btu/h | 186,735 |
| Electric kW | 44.9 |
| Electric amps | 187 |
Pick a model whose flow-vs-rise chart delivers at least this gpm at this rise. Use the winter inlet temperature (coldest groundwater) for the rise. Electric figures are the heat delivered to the water; confirm your service can carry the amps.
FAQ
What size tankless water heater do I need?
Add up the hot water you use at the same time, then find the rise. DOE's Building America example: 4.5 gpm peak, Seattle cold water about 52 °F, 120 °F setpoint = 68 °F rise. That is 4.5 × 8.34 × 60 × 68 ≈ 153,000 Btu/h of output, about 187,000 Btu/h of gas input at 82%.
How many gpm does a house need?
The DOE tool lists showers at 0.8 to 2.5 gpm (mixed hot and cold), whole-house recurring peaks of 3 to 4 gpm and severe peaks of 6 to 8 gpm. Count only the fixtures you expect to run together.
Why does my location matter?
Colder groundwater means a bigger rise, so the same unit delivers fewer gallons per minute in winter. Use your winter inlet temperature, not the summer one.
Can an electric tankless heater do this?
The same 153,000 Btu/h is about 45 kW, or about 187 amps at 240 V, which many homes cannot supply. A smaller peak or warmer inlet water lowers the kW needed; check the amps against your panel.
How do I choose a model?
Use the maker's flow-versus-temperature-rise chart: the unit must deliver your peak gpm at your rise. For gas, also check the gas line and venting.
Sources
- U.S. DOE Building America Solution Center (PNNL), Gas-Fired Tankless Water Heaters: size by the practical maximum coincident flow and the temperature rise from winter cold-water temperature to the setpoint (example: 4.5 gpm, Seattle ≈52 °F to 120 °F = 68 °F rise)
- U.S. DOE Building America Solution Center (PNNL), DHW Instantaneous Sizing tool: typical mixed flow rates (showers 0.8–2.5 gpm, whole-house recurring peak 3–4 gpm, severe peak 6–8 gpm; from the ASHRAE Handbook)
- USGS Circular 1268, conversion factors: 1 gallon of water = 8.34 pounds
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Last reviewed 2026-10-30.