Tankless water heater size calculator: what size tankless water heater do I need?
5 gpm at a 70°F rise
2 showers × 2.5 gpm · 50°F in, 120°F out · running at the same time
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
5 gpm at a 70°F rise
2 showers × 2.5 gpm · 50°F in, 120°F out
Source: U.S. Department of Energy, Building America, Measure Guideline: Transitioning to a Tankless Water Heater (IBACOS, September 2012), section 2.1 and Table 2 Typical Flow Rates, read November 7, 2026. Data sheet (PDF)
Two showers running at once need a tankless water heater that delivers 5 gpm at a 70°F rise, from 50°F incoming water to the 120°F the DOE gives for most uses, at 2.5 gallons per minute a shower.
A worked example
One 1.8 gpm shower, the kitchen sink and the dishwasher at once, from 42°F winter water to 115°F needs 6.2 gpm at a 73°F rise. 1.8 + 2.5 + 1.9 = 6.2 gpm, and 115 − 42 = a 73°F rise: the heater's flow chart has to show 6.2 gpm or more at 73°F.
What size tankless water heater for 2 bathrooms?
With both showers running, 5 gpm; add the kitchen sink and it is 7.5 gpm. The rise is your hot water temperature minus your coldest incoming water: 70°F from 50°F water to 120°F, 80°F from 40°F water.
What temperature rise do I need for a tankless water heater?
Hot water temperature minus incoming water temperature. The DOE guide's own example: with 65°F water and a 105°F shower, a 2.0 gpm shower needs a heater that gives a 40°F rise at 2.0 gpm.
What else it is called
Also searched as a tankless water heater size calculator, what size tankless water heater for a family of 4 or for 2 bathrooms, or a tankless water heater sizing chart. A family is sized by what runs at once, not by how many people: two showers and the dishwasher come to 6.9 gpm.
Flow by fixtures running at once
| Running at once | Hot water flow |
|---|---|
| 1 shower | 2.5 gpm |
| 1 shower + kitchen sink | 5 gpm |
| 2 showers | 5 gpm |
| 1 tub filling | 4 gpm |
| 2 showers + dishwasher | 6.9 gpm |
| 2 showers + kitchen sink | 7.5 gpm |
| 3 showers | 7.5 gpm |
| 1 shower + tub + laundry sink | 10.5 gpm |
DOE Building America, Measure Guideline: Transitioning to a Tankless Water Heater (2012), Table 2: shower 2.5 gpm, kitchen sink 2.5, laundry sink 4.0, dishwasher 1.9, bathtub 2.4 to 4.0 (taken at 4.0). Read the flow at your rise, the hot water temperature (120°F for most uses) minus the incoming water: 80°F from 40°F water, 70°F from 50°F, 60°F from 60°F.
Where the numbers come from
- DOE Building America, Measure Guideline: Transitioning to a Tankless Water Heater, 2012: “The size is based on the maximum temperature rise … needed and the maximum flow rate (gpm) demanded,” the rise running from the cold inlet to the hot outlet. “The desired output temperature is 120°F for most uses.” Table 2 (ASPE 2006): bathtub 2.4 to 4.0 gpm, shower 2.5, kitchen sink 2.5, laundry sink 4.0, dishwasher 1.9. Read November 7, 2026. DOE guideline
- 10 CFR 430.32(p): showerheads made after January 1, 1994, at most 2.5 gallons per minute at a flowing pressure of 80 psi; 430.32(o): lavatory and kitchen faucets 2.2 gpm at 60 psi. Read November 7, 2026. 10 CFR 430.32
- A tub is counted at 4.0 gpm, the top of its 2.4 to 4.0 range, because the heater is sized for the peak. The page gives the flow and the rise, the two numbers a maker's flow chart is read with, as the guide sizes by them.
The guide takes the incoming temperature from your water utility as a minimum value, so the coldest month sizes the heater. Faucets made since 1994 are capped at 2.2 gpm at 60 psi (10 CFR 430.32), under the 2.5 gpm the chart gives a kitchen sink.
By Javier Miralles · Last updated . How this site is made.
Other materials
Embed this calculator
Free to use on your own site. Copy the code and keep the credit line.