Tankless sizing — add up everything running at once, not one fixture
2 showers (1.5 GPM each) + a kitchen faucet (1.4 GPM) = 4.4 GPM peak demand. At a 68°F rise (52°F inlet to a 120°F setpoint), that needs 149,600 BTU/hr — buy on the GPM-vs-rise curve, not the headline max.
What this diagram shows
Three fixture icons — two showers at 1.5 gallons per minute each and a kitchen faucet at 1.4 gallons per minute — with arrows converging into a peak-demand total of 4.4 gallons per minute. A chain below multiplies that by 500 and by a 68-degree temperature rise, from a 52-degree incoming water temperature to a 120-degree setpoint, yielding a required output of 149,600 BTU per hour, about 43.9 kilowatts. A footer note warns that an advertised maximum GPM is measured at a smaller temperature rise than a real winter design condition.
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Tankless Water Heater Size Calculator
Free tankless water heater sizing calculator — peak GPM and temperature rise to the flow rate and BTU/kW you need, with WaterSense and Title 20 presets.
