Tankless water heater size calculator
A tankless water heater is sized by two numbers: the flow of hot water you need at one time, and how many degrees the heater must raise the incoming water. Enter both and the calculator gives the gas input or electric power required.
- Flow needed4.7 GPM
- Temperature rise70 °F
- Gas unit, input rating177,000 BTU/h
- Electric unit48.2 kW
Heat required: 500 × 4.7 GPM × 70 °F = 164,500 BTU/h delivered to the water.
A gas unit at 93% efficiency needs an input rating of about 177,000 BTU/h. An electric unit needs 48.2 kW, which draws about 200 amps at 240 volts.
Compare the flow figure with the GPM a model delivers at your temperature rise, which every manufacturer lists. A model is large enough when that figure is at or above the flow needed.
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BTU needed by flow and temperature rise
A tankless water heater is sized by two numbers. One is the flow of hot water you need at one time, in gallons per minute (GPM). The other is the temperature rise, the hot water temperature minus the incoming water temperature.
| Flow (GPM) | 40 °F rise | 50 °F rise | 60 °F rise | 70 °F rise |
|---|---|---|---|---|
| 2 | 40,000 | 50,000 | 60,000 | 70,000 |
| 3 | 60,000 | 75,000 | 90,000 | 105,000 |
| 4 | 80,000 | 100,000 | 120,000 | 140,000 |
| 5 | 100,000 | 125,000 | 150,000 | 175,000 |
| 6 | 120,000 | 150,000 | 180,000 | 210,000 |
| 8 | 160,000 | 200,000 | 240,000 | 280,000 |
BTU/h delivered to the water. A 70 °F rise heats 50 °F water to 120 °F. The US Department of Energy (DOE) says to assume 50 °F incoming water unless you know otherwise. It says 120 °F suits most uses.
DOE’s example runs a 0.75 GPM faucet and a 2.5 GPM shower head together. That is 3.25 GPM. At a 70 °F rise it takes 113,750 BTU/h.
kW needed for an electric unit
An electric unit is rated in kilowatts. One kilowatt is 3,412 BTU/h. This table gives the kilowatts needed for 1 to 4 GPM.
| Flow (GPM) | 40 °F rise | 50 °F rise | 60 °F rise | 70 °F rise |
|---|---|---|---|---|
| 1 | 5.9 | 7.3 | 8.8 | 10.3 |
| 2 | 11.7 | 14.7 | 17.6 | 20.5 |
| 3 | 17.6 | 22.0 | 26.4 | 30.8 |
| 4 | 23.4 | 29.3 | 35.2 | 41.0 |
DOE says a gas unit typically delivers about 5 GPM at a 70 °F rise and an electric unit about 2 GPM. By these tables, 5 GPM takes 175,000 BTU/h and 2 GPM takes 20.5 kW.
How the calculator works
The calculator adds the flow rates of the fixtures you say run at the same time, as DOE directs. It then applies this equation:
BTU/h = 500 × GPM × temperature rise (°F)
The 500 is 8.33 pounds per gallon × 60 minutes = 499.8, rounded. Dividing the result by 3,412 gives kilowatts. For gas, the calculator divides by an assumed 93% efficiency to show an input rating. That 93% is its own assumption, not a standard.
The fixture flows come from three kinds of source.
| Fixture | Flow (GPM) | Where the figure comes from |
|---|---|---|
| Showers | 2.5 | Federal maximum for new showerheads |
| Bathtubs filling | 4.0 | DOE Building America guideline, top of its range |
| Kitchen faucets | 2.2 | Federal maximum for new kitchen faucets |
| Bathroom faucets | 1.5 | WaterSense maximum for bathroom faucets |
| Dishwashers | 1.9 | DOE Building America guideline |
Limits of this calculator
- The fixture flows are full flows. They overstate the share that is hot water.
- Use the coldest incoming water. The Building America guideline says to use the coldest incoming temperature the utility supplies.
- Check the manufacturer’s flow-versus-rise table. A model is large enough when it delivers your flow at your rise.
- Small draws can fail to start a gas unit. A DOE-funded guide puts the typical minimum flow of a gas unit at 0.4 to 0.75 GPM.
Sources
- US Department of Energy, Energy Saver, Sizing a New Water Heater (archived copy of December 30, 2024): https://web.archive.org/web/20241230224527/https://www.energy.gov/energysaver/sizing-new-water-heater
- DOE Building America, Measure Guideline: Transitioning to a Tankless Water Heater: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_tanklesswh.pdf
- DOE-funded Building America strategy guideline for water heater selection, 2015: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/strategy-guideline-water-heater-selection.pdf
- eCFR, 10 CFR 430.32: https://www.ecfr.gov/current/title-10/section-430.32
- EPA WaterSense, bathroom faucets: https://www.epa.gov/watersense/bathroom-faucets
Related
For a tank water heater, use the water heater size calculator. Read how long a tankless unit lasts on the tankless water heater lifespan page, or date one with the water heater age lookup.
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Calculator last reviewed on October 5, 2026. Method and sources.