Tankless Water Heater Size Calculator
This tool starts with the coldest incoming water and the fixtures you may use at the same time. It shows the flow in GPM and the needed temperature rise. Use those two numbers to read a maker's performance chart.
1Choose what may run together
Count hot-water uses that may happen at the same time. Do not count every fixture in the house.
2Set the winter temperature rise
Use the electric model's rated conversion efficiency; resistance elements are typically close to 100%.
Highest hot-water flow at one time
5.0 GPM at a 70°F rise
Start by comparing units near the 5 GPM class. Then read each model's flow chart for this temperature rise.
Gas input required
184,211 BTU/h
Electric input required
51.8 kW
A large whole-house electric unit may need several high-amp circuits.
Temperature rise
70°F
50°F inlet to 120°F setpoint.
Choose a unit by the maker's delivered GPM at your winter temperature rise. Venting, gas pipe size, electrical service, minimum starting flow, recirculation, scale control, and the fixtures you use together may rule out a unit that looks large enough on the box.
Inputs stay in this browser. No account or quote request is required.
Quick planning answer
Tankless water heaters are sized by GPM at a certain temperature rise. Two 2.0 GPM showers and one bathroom faucet need about 5 GPM. Heating 50°F inlet water to 120°F is a 70°F rise. At 95% efficiency, that load needs about 184,000 BTU/h of gas input.
Count what may run at the same time
Do not add every fixture in the house. Add the fixtures your household may truly use at the same time. That might be two showers and a bathroom faucet, or one shower while the washer fills. A large tub filler creates a short, heavy load. You may choose a slower tub fill instead of buying a larger heater for that one use.
When possible, measure fixture flow with a marked container and a stopwatch. Low-flow showerheads can reduce both heater size and fuel use. Some older tub valves may use more water than the preset in this tool.
Winter water temperature changes the available GPM
The large GPM number on the box usually assumes a small temperature rise. The same burner supplies fewer gallons per minute when winter water enters near 40°F. Use the maker's flow chart at your full temperature rise, not the largest number on the box.
- Check the gas meter and pipe capacity at the heater's highest input.
- Plan an approved combustion-air and vent path. Condensing models also need a condensate drain.
- Check electrical demand for a whole-house electric tankless heater.
- Check minimum starting flow, recirculation controls, scale-service valves, and maintenance needs.
An electric tankless heater may require panel work
A whole-house electric unit at a high flow and a 70°F rise can use tens of kilowatts. It may need several double-pole breakers, large wires, and an electrical service upgrade. A small point-of-use heater for one sink is a much smaller load.
Gas tankless heaters can also need costly changes. A 199,000 BTU/h burner may be too large for the existing gas pipe or meter, even when the old tank heater worked well. Include those changes before comparing energy savings.
Questions people ask before buying
What size tankless water heater do I need?
Add the hot-water GPM of fixtures that may run together. Then subtract the coldest inlet temperature from the wanted outlet temperature. Choose a model that can supply that GPM at the full temperature rise.
How many GPM for two showers?
Two modern 2.0 GPM showerheads need about 4 GPM of mixed water. Add any faucets or appliances that may run at the same time. Then check the heater's flow at the winter temperature rise.
Is a 199,000 BTU tankless heater enough?
It often supports a strong whole-home flow. However, available GPM drops as temperature rise goes up. Check the exact model chart, gas supply, venting, altitude adjustment, and the fixtures your household may use together.
Can my 200-amp panel support electric tankless?
Maybe. Large whole-house models can use 18 to 36 kW or more. A formal load calculation and the exact circuits required by the model will show whether the service has enough capacity.
Method, sources and limits
The presets use 2.0 GPM for each shower, 1.0 GPM for a bathroom faucet, 1.5 GPM for a kitchen faucet or dishwasher, 2.0 GPM for a clothes washer, and 4.0 GPM for a tub filler. Temperature rise equals outlet temperature minus inlet temperature.
Needed heat equals 500 × GPM × temperature rise in BTU/h. The tool divides that amount by the gas thermal efficiency for required gas input. It separately divides by 3,412 and the electric conversion efficiency to show electric kW. Choose the final unit from the maker's flow-versus-temperature-rise chart.
- Transitioning to a tankless water heater (U.S. Department of Energy Building America)
- Gas-fired tankless water-heater basics (U.S. Department of Energy Building Science Education)
- Consumer water-heater definitions and standards (U.S. Department of Energy)