A tankless water heater is sized by two numbers: how many gallons per minute (GPM) of hot water you draw at once, and how many degrees it must heat that water. This calculator adds up the fixtures you run together and works out the temperature rise from your incoming water, using the method Navien and Rinnai publish. To compare the brands themselves, see tankless water heater brands.
This calculator is for estimating only — have a licensed plumber confirm sizing. Gas line size, venting and electrical supply decide what can actually be installed.
How to size a tankless water heater: add up the gallons per minute of every fixture that runs at the same time. Then subtract your incoming water temperature in the coldest months from the output setting to get the temperature rise; Navien’s example is 120°F − 55°F = 65°F. Choose a model whose flow-versus-rise chart shows at least your total GPM at that rise. Two 2.5 GPM showers at a 65°F rise need 5 GPM.
How the calculator works
Flow (GPM) = the sum of (number of each fixture × its gallons per minute), for everything running at the same time. Navien’s example: a dishwasher (2 to 3 GPM), a shower (2 to 3 GPM) and a hot-water washing machine (3 to 5 GPM) together need 7 to 11 GPM.
Temperature rise (°F) = output temperature − incoming water temperature. Navien’s example: 120°F − 55°F = a 65°F rise. Navien says to use the coldest days of the year, because incoming water is coldest then.
Heat into the water (Btu per hour) = GPM × 60 minutes × 8.34 pounds per gallon × rise. A Btu warms one pound of water by 1°F (EIA) and a gallon of water weighs 8.34 pounds (USGS). Divide by 3,412 to get kilowatts (EIA: 1 kWh = 3,412 Btu).
The starting flow rates are editable defaults: 2.5 GPM for a shower is the federal maximum for showerheads made after 1994 (at 80 psi), 0.75 GPM for a faucet is A. O. Smith’s rough guide (the federal maximum for faucets is 2.2 GPM at 60 psi), and the dishwasher and washer figures are the low end of Navien’s ranges (2 to 3 GPM and 3 to 5 GPM). For a margin, enter the high end, or your own fixtures’ rated flow.
Navien’s rough guide, for comparison:
| Fixtures running at once | Rough flow needed |
|---|---|
| 1 to 2 | 3.5 GPM |
| 2 to 3 | 5 GPM |
| 3 to 4 | 7 GPM |
Worked examples
| Running at the same time | Incoming water | Look for | Heat into the water |
|---|---|---|---|
| 1 shower (2.5) + 1 bathroom faucet (0.75) | 55°F | 3.25 GPM at a 65°F rise | 105,700 Btu/h |
| 2 showers (2.5 each) | 55°F | 5 GPM at a 65°F rise | 162,600 Btu/h |
| 2 showers (2.5 each) + 1 kitchen faucet (0.75) | 40°F | 5.75 GPM at an 80°F rise | 230,200 Btu/h |
| 1 shower (2.5) + dishwasher (2) + hot-water washer (3) | 55°F | 7.5 GPM at a 65°F rise | 243,900 Btu/h |
The last column is the heat the water needs, not a heater’s input rating. Compare the flow and rise to the maker’s chart.
Limits of this estimate
- Real fixture flow varies with the fixture and your water pressure. Navien notes that pipe size and the incoming supply can also limit flow.
- Incoming water temperature changes with the season and region. Rinnai’s method starts from the groundwater temperature where you live.
- Navien says tankless capacity is rated by gas energy input in Btu per hour. The Btu figure here is the heat the water itself needs, so do not match it one-to-one to an input rating. Compare your GPM and rise to the maker’s chart.
- This sizes one heater for the whole load. It does not size the gas line, venting or electrical service; those need a licensed plumber and, for electrical, a licensed electrician.
FAQ
What is temperature rise on a tankless water heater?
Rinnai defines it as the difference between the heater’s set temperature and the incoming water temperature. Set to 120°F with 60°F incoming water, the rise is 60°F. A bigger rise means less hot water flow from the same heater.
How many GPM does a tankless water heater need for a family?
It depends on what runs at once, not on how many people live there. Navien’s rough guide is 3.5 GPM for 1 to 2 fixtures at once, 5 GPM for 2 to 3 and 7 GPM for 3 to 4, checked against the rise in the coldest months.
Why does the calculator show Btu per hour?
One Btu warms one pound of water by 1°F (EIA), and a gallon of water weighs about 8.34 pounds (USGS). The Btu figure shows how much heat your flow and rise call for, which is why a larger rise needs a larger heater.
Related guides
- Best tankless water heaters
- Tank water heater size calculator
- Heat pump vs tankless water heater
- How to descale a tankless water heater
Sources
- Navien, “Tankless Water Heater Sizing Guide” (Sept. 13, 2024)
- Rinnai, Residential FAQ (“What is temperature rise?” and “What size tankless water heater do I need?”)
- A. O. Smith, “What Size Water Heater Is Right for You?”
- 10 CFR 430.32, federal water conservation standards for showerheads and faucets (eCFR)
- U.S. Energy Information Administration, “British thermal units (Btu)”
- USGS Water Science School, “A Million Gallons of Water — How much is it?”
Sources checked September 24, 2026.