What size tankless water heater do I need?
Tankless heaters are sized by flow (GPM) at your temperature rise. Two showers and a kitchen faucet at once need about 5.5 GPM. Pick the fixtures you run together below.
Method: U.S. Department of Energy First Hour Rating method for tanks; flow × temperature rise × 500 for tankless.
Fixtures running at the same time
Recommended tankless capacity
5.5 GPM
at a 70°F temperature rise (to 120°F)
| Heat output needed | 192,500 BTU/h |
| Gas unit input rating (95% efficient) | 203,000 BTU/h |
| Electric unit size | 56.4 kW |
- • You need a tankless heater that delivers 5.5 GPM at a 70°F temperature rise. Check the maker's flow-vs-rise chart, not just its maximum GPM.
- • This is more than most whole-house electric tankless units supply. A gas unit, or two electric units, may be needed.
- • This is above the largest residential gas tankless units (about 199,000 BTU), so you may need two units.
How water heater size is calculated
The U.S. Department of Energy recommends sizing a storage tank by its First Hour Rating, the gallons of hot water it can deliver in one busy hour starting full. Estimate your household's peak hour demand, then choose a heater whose First Hour Rating, printed on its EnergyGuide label, at least matches it.
| Use | Hot water per use |
|---|---|
| Showers | 20 gallons |
| Baths | 20 gallons |
| Shaving / hand washing | 2 gallons |
| Hand dishwashing / food prep | 4 gallons |
| Dishwasher loads | 7 gallons |
| Laundry loads (top-loader) | 25 gallons |
| Laundry loads (front-loader) | 15 gallons |
Tankless heaters are sized by flow instead: add up the gallons per minute of every fixture you run at the same time, and find your temperature rise from incoming water to 120°F. The heat needed in BTU per hour is flow (GPM) × rise (°F) × 500.
Water heater size by household
| People | Typical tank size |
|---|---|
| 1–2 | 30–40 gallons |
| 2–4 | 40–50 gallons |
| 5 | 50–55 gallons |
| 6+ | 65–80 gallons or tankless |
More water heater guides
Frequently asked questions
What size water heater for a family of 4?
Usually a 40–50 gallon tank. By DOE's method, a family of 4 with 3 showers in the busiest hour needs a First Hour Rating of about 68 gallons, which a 40 gallon gas or 65 gallon electric heater typically meets.
What is a First Hour Rating?
It's how many gallons of hot water a tank heater can supply in one hour, starting full. It's on the yellow EnergyGuide label and matters more than tank size.
Why do gas tanks deliver more than electric?
Gas burners reheat water faster, so a 40 gallon gas tank often has a higher First Hour Rating than a 50 gallon electric one.
What size tankless water heater do I need?
For two showers and a kitchen faucet at once, about 5.5 GPM. In a cold climate with 40°F incoming water, that means roughly 220,000 BTU/h of heat, which is near the top of residential gas units.
What temperature should a water heater be set to?
The Department of Energy recommends 120°F for most homes. It saves energy and reduces scalding risk.