Tankless Water Heater Sizing Calculator
Size an on-demand water heater from simultaneous fixture flow and the temperature rise from your groundwater temperature: required BTU/h input for gas units (with efficiency), kW and amperage for electric units, and the flow a given unit delivers at your temperature rise.
How it works
BTU/h delivered = gpm × 500 × ΔT (°F); input = delivered ÷ efficiency; kW = BTU/h ÷ 3,412; amps = kW × 1000 ÷ 240; unit gpm = rated input × efficiency ÷ (500 × ΔT)
A tankless heater has no reserve, so it must heat water as fast as it is drawn, and the power required is simply flow times temperature rise: a gallon per minute is 500 BTU per hour per degree Fahrenheit. That makes the incoming water temperature the deciding variable — the same two showers that need a 45 °F rise in Florida need 75 °F in Minnesota in January, nearly twice the heater. Gas units of 150,000–199,000 BTU/h handle a typical house; electric units need enormous currents (a 27 kW heater draws over 110 A) because 3,412 BTU/h is only 1 kW, which is why whole-house electric tankless is often impractical on a 200 A service and point-of-use units are used instead.
Worked example
Two 2.0 gpm showers plus a 1.5 gpm faucet is 5.5 gpm; heating 50 °F groundwater to 120 °F is a 70 °F rise, so 192,500 BTU/h must be delivered — a 203k input condensing gas unit, or 56 kW electric (235 A). A 199k BTU/h condensing unit gives 5.4 gpm at that rise, just short, but 7.6 gpm in a southern climate with 50 °F rise.
Frequently asked questions
Why does the manufacturer's gpm rating look higher?
Ratings are usually quoted at a 35 °F rise (e.g. "11 gpm"). Divide by your real rise ratio — at 70 °F the same unit gives half that.
Should I size for every fixture at once?
No — size for realistic simultaneous use (two showers, or a shower plus a sink). A small buffer tank or recirculation loop is a better fix for occasional peaks than a much larger burner.
Related calculators
Simplified sizing using common US plumbing-code tables (IPC/UPC style) and standard heat-transfer arithmetic. Local codes, listed product data and a licensed plumber or engineer govern the final design; verify pressure, temperature and fixture requirements on site.