Compare the total with the FHR on each heater's EnergyGuide label and pick one that meets or slightly exceeds it.
How the water heater size calculator works
Tank heaters are sized by their first-hour rating (FHR): how many gallons of hot water they deliver in one busy hour, starting with a full tank. WaterHeaterSize adds up your peak-hour demand with the US Department of Energy worksheet figures per use: shower 10 gal, bath 20, shaving 2, hand dishwashing or food prep 4, dishwasher 6 and clothes washer 7. Pick a heater whose FHR is at least that total. Tankless heaters are sized by flow and temperature rise instead: the flow of every fixture that may run at once (shower 2.0 GPM, bathroom faucet 1.0, kitchen faucet 1.5, tub 4.0, dishwasher 1.0, washer 2.0) times the rise from the incoming water to the setting. Heat needed = GPM × 500 × rise in °F, in BTU per hour; divide by efficiency for gas input, or by 3,412 for electric kW.
Tankless size by flow at a 70°F rise (50°F water heated to 120°F)
| Flow | Heat output | Gas input at 95% | Electric | Amps at 240 V |
|---|---|---|---|---|
| 1 GPM | 35,000 BTU/h | 36,842 BTU/h | 10.3 kW | 43 A |
| 2 GPM | 70,000 BTU/h | 73,684 BTU/h | 20.5 kW | 85 A |
| 3 GPM | 105,000 BTU/h | 110,526 BTU/h | 30.8 kW | 128 A |
| 4 GPM | 140,000 BTU/h | 147,368 BTU/h | 41 kW | 171 A |
| 5 GPM | 175,000 BTU/h | 184,211 BTU/h | 51.3 kW | 214 A |
| 6 GPM | 210,000 BTU/h | 221,053 BTU/h | 61.5 kW | 256 A |
Groundwater is colder in northern climates in winter (often 40°F or less), which raises the rise and the size you need. Low-flow fixtures reduce it. Gas units need correctly sized gas lines and venting, and electric units need a dedicated circuit; have a licensed installer confirm the final choice. Results are estimates for planning.
Frequently asked questions
How is peak-hour demand calculated?
Number of showers × 10 gal + baths × 20 + shaves × 2 + hand dishwashing × 4 + dishwasher loads × 6 + washer loads × 7, for the busiest hour.
Is first-hour rating the same as tank size?
No. It includes the water heated during the hour, so it is usually larger than the tank volume, more so for gas heaters.
Where do I find the first-hour rating?
On the yellow EnergyGuide label and in the product specifications, listed as FHR or first hour delivery.
Is WaterHeaterSize free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.