Tankless Flow Calculator
Work out the flow rate and temperature rise a tankless water heater has to deliver for your home, and see whether that is a small, standard, large or two-unit setup.
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Data: DOE water heater guidance (120°F setpoint) · federal and EPA WaterSense fixture flow rates · groundwater temperature bands from local winter climate (Daymet 2015–2024) · Updated September 2026
How tankless sizing works
Flow: what runs at the same time
A tank stores hot water; a tankless unit makes it as you use it, so it is sized by gallons per minute, not gallons. Add up the fixtures that realistically run together: a showerhead is about 2.5 GPM, a faucet about 1.8, a dishwasher or washer about 2. Two showers at once already means 5 GPM.
Rise: how cold the water comes in
The unit has to lift incoming water to the setpoint, and DOE recommends 120°F. Ground water in a cold-winter state can arrive in the low 40s in January, a rise of nearly 80°F; in a warm climate it is in the 60s and the rise is closer to 55°F. Same house, very different job.
Flow times rise is the size
Heat needed per hour is GPM times rise times 500 (from the weight of a gallon of water). A tankless unit can trade one for the other: the same unit that gives 8 GPM at a small rise gives 4 or 5 GPM at a big one, which is why manufacturer charts are plotted by rise.
Gas versus electric capacity
Gas units go up to about 199,000 BTU per hour input, enough for a whole house in most climates, and can be linked in pairs. Electric units top out near 36 kW, about 123,000 BTU per hour, and need a very large electrical service, so in cold-groundwater areas they cover roughly one shower at a time.
Common questions
Usually the flow for two showers at once, about 5 GPM, at your local temperature rise. In a warm climate that is a standard unit; in a cold-winter state it lands on a large unit or two linked units. The calculator gives the class for your climate.
A large gas unit can, in most climates. Two showers is about 5 GPM, and at a 55 to 70°F rise that is within reach of one high-capacity unit. At the 78°F rise of a cold-groundwater state it is at the edge, which is where two units or low-flow showerheads come in.
The difference between the incoming water temperature and the setpoint. With 120°F out and 45°F in, the rise is 75°F. Every tankless flow rating is quoted at a rise, so a unit rated 9 GPM at 35°F rise gives far less in a cold climate.
Rarely in cold climates. The largest common electric units, about 36 kW, deliver around 3 GPM at a 78°F rise, roughly one shower. In warm-groundwater areas the same unit reaches 4 to 5 GPM. Either way it needs a very large electrical service, which an electrician has to confirm.
Often. A 199,000 BTU per hour unit draws several times what a tank heater does, and the existing gas line and venting may not be sized for it. That is one of the first things a plumber checks on a tankless conversion.
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