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Water Heater Size Calculator: Tank or Tankless
Size a new water heater the way the US Department of Energy recommends: by your busiest hour of hot water use for a tank, and by flow rate and temperature rise for a tankless unit.
Sizing a tank water heater
A tank's gallon size is less important than its first-hour rating (FHR): how many gallons of hot water it can deliver in an hour starting with a full tank. It is printed on the yellow EnergyGuide label. Add up the hot water you use in your busiest hour and choose a heater whose FHR matches or beats that number; the calculator adds about 10% headroom.
| Use | Hot water, about |
|---|---|
| Shower | 10 gal |
| Bath | 20 gal |
| Dishwasher load | 6 gal |
| Clothes washer, warm | 7 gal |
| Food prep, hand dishwashing or shaving | 2 to 5 gal |
Example: three showers, a dishwasher load, a load of laundry and two sink tasks on a busy morning add up to about 51 gallons. A typical 40 gallon gas heater (FHR around 70) covers that, while an electric tank needs to be about 50 gallons, because electric elements reheat water more slowly.
Sizing a tankless water heater
A tankless heater must heat water as fast as you draw it. Add the flow of everything that might run at once (about 2 gal/min per shower and 1 gal/min per faucet), then find the temperature rise: 120°F minus your incoming water temperature, which ranges from about 40°F in northern winters to 70°F in the south.
gas input (BTU/h) ≈ gal/min × 500 × temperature rise ÷ efficiency
Two showers and a faucet at once need 5 gal/min. With 50°F incoming water that is a 70°F rise, so about 185,000 BTU/h, which is why whole-house gas tankless units are usually rated 180,000 to 199,000 BTU/h. An electric unit would need about 52 kW, more than most homes' electrical service can supply.
Rules of thumb by household size
| People | Gas tank | Electric tank | Tankless flow |
|---|---|---|---|
| 1 to 2 | 30 to 40 gal | 40 gal | 4 to 5 gal/min |
| 2 to 4 | 40 to 50 gal | 50 gal | 5 to 7 gal/min |
| 4 to 5 | 50 gal | 65 to 80 gal | 7 to 9 gal/min |
| 5 or more | 75 gal | 80 gal | 9 gal/min or two units |
Which type is cheapest to run?
- Heat pump water heaters use about a third of the electricity of a standard electric tank and often qualify for rebates and tax credits. They need a space that stays above about 40°F with room around the unit.
- Gas tanks are the cheapest to buy and recover quickly.
- Tankless heaters never run out and save standby losses, but cost more to install and need a gas line and venting sized for them.
To estimate what a water heater costs to run, try the electricity cost calculator.
Frequently asked questions
What size water heater do I need for a family of 4?
Usually a 50 gallon gas tank, or a 50 to 65 gallon electric or heat pump tank. For tankless, plan on 6 to 8 gallons per minute at your winter temperature rise.
What is first-hour rating?
The first-hour rating is how many gallons of hot water a tank heater can supply in one hour starting full. It is on the EnergyGuide label and is the best number for comparing tanks.
How many gallons per minute does a tankless water heater need?
Add about 2 gal/min for each shower and 1 gal/min for each faucet that may run at once. Two showers and a sink need about 5 gal/min.