Tankless water heater size calculator

A tankless water heater has to heat water as fast as it flows. Two numbers decide the size you need: how many gallons per minute (GPM) run at once, and how many degrees the heater must raise the incoming water, called the temperature rise.

The calculator above adds up typical fixture flow rates and uses the winter ground water temperature for your region, because that is when a tankless heater works hardest.

How the numbers are calculated

Heating one gallon of water by one degree Fahrenheit takes about 8.3 BTU. At a flow of one gallon per minute, that works out to roughly 500 BTU per hour for each degree of rise. The calculator multiplies your flow rate by your temperature rise and by 500, then divides by the heater's efficiency to get the gas input needed.

For electric units, the same heat is converted to kilowatts: flow in GPM times temperature rise, divided by about 6.83.

Why winter matters

Ground water can be 70°F in south Florida and close to 40°F in the northern states in winter. Heating water to 120°F means a 50°F rise in the first case and an 80°F rise in the second. The same heater delivers far less hot water in the colder case, which is why a unit that works in Texas can disappoint in Minnesota.

Gas or electric?

Gas tankless heaters typically deliver more flow, but often need a larger gas line and new venting. Electric tankless heaters avoid venting, but whole-house units can draw well over 100 amps, which may mean new circuits or a panel upgrade. Our guide to tankless water heater installation cost explains what those upgrades add.

Tips for choosing a size

• Check the manufacturer's flow chart at your temperature rise, not the maximum GPM on the box.
• Low-flow showerheads reduce the size you need.
• If you are close to the limit, size up; an undersized unit shows up as lukewarm showers in winter.