Estimate tankless water heater size from simultaneous fixture flow, temperature rise, incoming water temperature, target hot water temperature, required GPM, BTU per hour, kilowatts, and known heater capacity.
Fixture demand:
The calculator adds showers, sinks, appliances, and extra fixtures that may run at the same time.
Temperature rise:
The calculator subtracts incoming water temperature from target hot water temperature.
BTU and kW estimate:
The calculator estimates heating power needed from GPM, temperature rise, and heater efficiency.
Capacity check:
The calculator compares a known tankless heater capacity against the estimated required GPM.
A tankless water heater size calculator helps estimate whether a tankless heater can supply the hot water flow needed at the required temperature rise.
It can help compare fixture demand, required GPM, incoming water temperature, target temperature, BTU/hr, kW, heater efficiency, and capacity margin.
Your result shows estimated required tankless GPM, temperature rise, BTU/hr, kW, and capacity margin. This is a planning estimate only. Final tankless water heater sizing should be checked against manufacturer sizing charts, fuel type, venting, electrical capacity, gas line capacity, local code, and actual fixture flow rates.
Add the flow rates of fixtures that may run at the same time, then choose a tankless heater that can provide that GPM at your required temperature rise.
Temperature rise is the difference between incoming cold water temperature and the desired hot water temperature.
The required GPM depends on simultaneous fixture use. One shower may need about 2 to 3 GPM, while a whole-house system may need much more.
No. This calculator gives a planning estimate. Final sizing should be checked against manufacturer charts and local gas, electrical, venting, and plumbing requirements.