Tankless Water Heater Sizing Calculator

You size a tankless heater on two numbers at once: the peak flow rate in gallons per minute (GPM) and the temperature rise in degrees Fahrenheit. Add up the GPM of every fixture you expect to run at the same time, then subtract the incoming water temperature from your target setpoint to get the rise. The unit you buy must deliver that GPM at that rise, which is exactly what this calculator solves for.

The heat output uses the sensible-heat equation for water. Raising one gallon per minute by one degree Fahrenheit takes about 500.4 BTU per hour, which comes from 8.34 pounds per gallon times 60 minutes per hour times 1 BTU per pound per degree. So the required output equals your GPM times the temperature rise times 500.4. For example, 5 GPM at a 70 degree rise needs 5 times 70 times 500.4, which is 175,140 BTU per hour delivered.

The rise is your target setpoint minus the incoming groundwater temperature. Most households set hot water to 120 F. Incoming water runs about 40 F in northern states in winter and 70 F or higher in the south, so the same heater delivers fewer gallons per minute in cold regions because it works harder to close a bigger gap. Use your local winter groundwater temperature for a worst-case size.

Electric tankless units are simpler to install and vent but top out near 36 kW, which limits them to roughly one or two simultaneous fixtures at a high temperature rise. Gas tankless units reach much higher inputs, often 150,000 to 199,000 BTU per hour, so they handle whole-home demand and cold inlet water more easily. This calculator recommends electric when the required kW stays under the 36 kW ceiling and gas when it does not.

No heater converts 100 percent of its fuel into delivered heat. A non-condensing gas tankless is about 82 percent efficient, so its input rating must be the delivered output divided by 0.82. Electric resistance heating is about 99 percent efficient, so its input is nearly the same as the output. That is why the gas BTU per hour figure shown is larger than the required output, while the electric kW figure tracks the output closely.

No. It is a planning estimate that uses standard flow rates and the sensible-heat formula. Before buying, confirm the manufacturer's published GPM-at-rise chart for the specific model, and verify that your gas line and meter or your electrical service can supply the rated input. A licensed plumber or electrician should size the supply for the final install.