About the Tankless Water Heater Size Calculator
This tankless water heater size calculator works out how big an on-demand heater must be to keep up with the hot water you use at the same time. Enter how many showers, faucets and appliances might run at once and their flow rates, plus your incoming water temperature and the hot water temperature you want, and it returns the peak flow in gallons per minute, the temperature rise, and the gas input (BTU/h) or electric power (kW and amps) needed.
Tankless heaters are rated by flow at a given temperature rise, and rise is the part people underestimate. Groundwater in northern US states can be around 40°F in winter, so heating it to 120°F needs an 80°F rise, while in warm southern climates the incoming water may be near 70°F. The same heater delivers far fewer gallons per minute in a cold climate.
The calculation treats each fixture flow as fully hot water, a conservative assumption: showers mix in some cold water, so the real hot-water draw is usually 60–80% of the fixture rating. You can also enter the input rating of a heater you are considering to see how many GPM it can supply at your rise.
With the default inputs, the peak flow needed is 5 GPM. Change any value above to recalculate instantly.
How to use the tankless water heater size calculator
- 1Enter your coldest-season incoming water temperature and the hot water temperature you want.
- 2List how many showers, faucets and appliances could run at the same time.
- 3Adjust each flow rate to match your fixtures (check shower heads for a GPM rating).
- 4Read the peak GPM and the gas BTU/h or electric kW needed.
- 5Enter a heater’s BTU rating to check whether it keeps up at your rise.
Formula and method
Heating one gallon of water by 1°F takes about 8.33 BTU, the weight of a gallon in pounds. A flow of one gallon per minute is 60 gallons per hour, so the heat output needed is 8.33 × 60 ≈ 500 × GPM × temperature rise, in BTU per hour.
Dividing by the heater efficiency gives the gas input rating, and dividing the output by 3,412 BTU per kWh gives electric kilowatts; amps are watts ÷ supply voltage. Turning the formula around, a heater of a given input can supply input × efficiency ÷ (500 × ΔT) GPM. Peak flow assumes every listed fixture runs at once with fully hot water, which is a conservative design case.
- GPM
- Peak simultaneous hot-water flow (gallons per minute)
- ΔT
- Temperature rise (°F)
- η
- Heater efficiency (decimal)
- 500
- 8.33 lb/gal × 60 min/h
Worked examples
Two showers and a bathroom faucet, 50°F inlet
Two 2.0 GPM showers plus a 1.0 GPM faucet need 5 GPM. Heating it from 50°F to 120°F takes 500 × 5 × 70 = 175,000 BTU/h of output, or about 184,000 BTU/h input at 95% efficiency. A 199,000 BTU/h unit supplies 5.4 GPM at this rise, so it keeps up.
Warm climate: one shower and the kitchen sink
A 1.8 GPM shower and a 1.5 GPM kitchen faucet need 3.3 GPM. With 70°F incoming water the rise is just 50°F, so about 24.4 kW of electric heating (102 A at 240 V) or 86,800 BTU/h of gas would do.
Cold northern winter: three showers and a washer
Three 2.5 GPM showers and a 2.0 GPM washer total 9.5 GPM at an 80°F rise, which needs 380,000 BTU/h of output — 400,000 BTU/h input. One 199,000 BTU/h unit gives only 4.7 GPM here, so two units in parallel would be needed.
Frequently asked questions
What size tankless water heater do I need?+
Add the flow rates of all fixtures you might use at once to get peak GPM, then find the temperature rise from your incoming water to about 120°F. Choose a unit rated for at least that GPM at that rise; manufacturers publish flow-versus-rise charts.
How many GPM does a shower use?+
US federal rules cap new shower heads at 2.5 GPM at 80 psi, and WaterSense-labeled heads use 2.0 GPM or less. Bathroom faucets are typically 1.0–1.5 GPM and kitchen faucets about 1.5–2.2 GPM.
What is temperature rise for a tankless water heater?+
Temperature rise is the difference between the incoming cold water and the hot water setting. With 50°F groundwater and a 120°F setting the rise is 70°F. The higher the rise, the fewer gallons per minute a given heater can deliver.
Can an electric tankless heater supply a whole house?+
It can in warm climates, but a whole-house electric unit often needs 27–36 kW or more, drawing well over 100 A at 240 V and usually several dedicated breakers. In cold climates gas or a heat-pump tank is often more practical.
Is a 199,000 BTU tankless heater enough?+
At 95% efficiency a 199,000 BTU/h unit supplies about 5.4 GPM at a 70°F rise, roughly two showers and a faucet at once. At an 80°F winter rise in the north it drops to about 4.7 GPM.