Tankless water heater Btu/h = GPM × temperature rise × 500 ÷ efficiency. Two showers and a kitchen sink running at once (6.5 GPM) with 55 °F incoming water need a 65 °F rise, which is 211,166 Btu/h into the water and about 222,000 Btu/h of gas input at 0.95 efficiency, or 63 kW of electric power.
How tankless water heater sizing works
A tankless heater has no storage, so its size is the flow it can heat, not gallons. Two numbers set it: how many gallons per minute (GPM) you run at once, and the temperature rise, the gap between the cold water entering and the hot water you want.
temperature rise = set temperature − incoming water temperature
Btu/h into the water = GPM × rise × 8.33 lb per gal × 60 min per h (about 500 × GPM × rise)
gas input = Btu/h into the water ÷ efficiency; electric kW = Btu/h ÷ 3,412 ÷ efficiency
The set temperature defaults to 120 °F, the safe setting the Consumer Product Safety Commission recommends. Size for winter, when the groundwater is coldest and you use the most hot water.
Worked example: two showers and a kitchen sink
| Step | Calculation | Result |
|---|---|---|
| Flow | 2 showers × 2.5 GPM + 1 kitchen sink × 1.5 GPM | 6.5 GPM |
| Rise | 120 °F − 55 °F (Mid-Atlantic winter) | 65 °F |
| Heat into the water | 6.5 × 65 × 499.8 | 211,166 Btu/h |
| Gas, condensing (0.95) | 211,166 ÷ 0.95 | 222,279 Btu/h |
| Gas, standard (0.82) | 211,166 ÷ 0.82 | 257,519 Btu/h |
| Electric (0.98) | 211,166 ÷ 3,412 ÷ 0.98 | 63.2 kW |
That electric figure is 263 A at 240 V, more than the 200 A service many homes have, so a whole-house electric unit only suits small loads or warm-water climates. Check the wire size and panel before you choose one.
Tankless water heater GPM by fixture
The calculator counts each fixture at the top of its range so the unit is not undersized:
| Fixture | GPM used | Typical range |
|---|---|---|
| Shower | 2.5 (2.0 with low-flow heads) | 2.0 to 2.5 |
| Bathtub fill | 4.0 | 4.0 to 6.0 |
| Bathroom sink | 1.0 | 0.5 to 1.0 |
| Kitchen sink | 1.5 | 1.5 to 2.0 |
| Dishwasher | 1.5 | 1.0 to 1.5 |
| Clothes washer | 2.0 | 1.5 to 2.0 |
Count only what really runs together. Two showers and a dishwasher at once is 6.5 GPM; a tub fill plus a shower is 6.5 GPM too. Use the "anything else" field for a hot tub fill or a utility sink.
Incoming water temperature by state
Pick your state and the calculator fills the winter groundwater temperature from this regional table (Tankless Geek):
| Region | Winter water | States in this calculator |
|---|---|---|
| Far north | 40 °F | AK |
| Northern states | 45 °F | ME, NH, VT, MA, CT, RI, NY, MI, WI, MN, ND, MT |
| Pacific Northwest | 52 °F | WA, OR |
| Midwest and Great Plains | 50 °F | SD, NE, IA, IL, IN, OH, KS, MO, WY, CO, ID, UT |
| Mid-Atlantic | 55 °F | PA, NJ, DE, MD, DC, VA, WV |
| Southern states | 58 °F | KY, TN, NC, SC, OK, AR, TX |
| California | 58 °F | CA |
| Southwest desert | 60 °F | AZ, NV, NM |
| Deep South | 62 °F | GA, AL, MS, LA |
| Florida and Gulf Coast | 68 °F | FL, HI |
States the source table does not name are placed in the nearest climate group. Water temperature varies inside a state and with the season, so overwrite the number if you know yours. Minnesota at 45 °F is a 75 °F rise: two showers (5 GPM) need 187,425 Btu/h into the water, about 197,000 Btu/h of gas input. Florida at 68 °F is a 52 °F rise: the 6.5 GPM example needs 168,932 Btu/h, about 178,000 Btu/h of input.
GPM at each temperature rise
A heater's rating is a fixed heat output, so the flow it delivers falls as the rise grows. At 0.95 efficiency:
| Temperature rise (°F) | 40 | 50 | 60 | 70 | 80 |
|---|---|---|---|---|---|
| 150,000 Btu/h unit (GPM) | 7.1 | 5.7 | 4.8 | 4.1 | 3.6 |
| 200,000 Btu/h unit (GPM) | 9.5 | 7.6 | 6.3 | 5.4 | 4.8 |
If your GPM at your rise is more than one unit delivers, either drop a fixture from the "at once" list or plan two units.
The next questions
- What does the swap cost? See the water heater replacement cost for tank and tankless.
- Gas unit? Size the line with the gas pipe size calculator and convert ratings with the BTU calculator.
- If your water is hard, put a water softener in the plan before the heater.
- See every heating, cooling and plumbing calculator.