🚿Tankless Water Heater Size Calculator
Add up the fixtures you use at the same time and we'll tell you the flow rate and power a tankless heater needs.
Hot water at the same time
You need about
273,684 BTU/hr gas
6.5 GPM at a 80°F rise
GPM
6.5
temp rise °F
80
gas BTU/hr
273,684
electric kW
76.2
Size for winter, when groundwater is coldest. Big electric units often need 2–3 dedicated 240V circuits — check your panel first.
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How it works
- Flow rate (GPM) = sum of fixtures running at once (shower 2.5, faucet 1.5, dishwasher 1.5, washer 2.0 GPM by default).
- Temperature rise = desired hot water temp − groundwater temp.
- Gas: BTU/hr ≈ 500 × GPM × rise ÷ efficiency. Electric: kW ≈ 0.1465 × GPM × rise.
Worked example
Two showers and one faucet in the North (40°F groundwater), 120°F hot water:
- 2 × 2.5 + 1.5 = 6.5 GPM, rise = 80°F
- Gas at 95%: 500 × 6.5 × 80 ÷ 0.95 = 273,684 BTU/hr
- Electric: 0.1465 × 6.5 × 80 = 76 kW (too big for most homes — gas fits better)
Questions people ask
Add the flow of every fixture you might run at the same time. Most families need 5–8 GPM.
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Last updated October 8, 2026. These results are friendly estimates to help you plan — always double-check before buying or making big decisions.