What Size Water Heater Do I Need?
What size water heater do I need? For a tank, you need one whose first-hour rating covers the hot water your household uses in its single busiest hour.
For a tankless unit, you need one that can heat the flow of every fixture you run at once. Gallons on the label and the number of people in the house are only a rough starting point.
The quick answer by household size
If you just want a ballpark, Rheem's guidelines for tank size are a fair place to start:
| Household | Tank size |
|---|---|
| 1 to 2 people | 30 to 40 gallons |
| 2 to 4 people | 40 to 50 gallons |
| 4 to 5 people | 50 to 55 gallons |
| 5 or more people | 75 gallons, or tankless |
Treat that as a sanity check, not the answer. Two people who both shower first thing in the morning and run the dishwasher need more than two people who shower at different times. That's why the rating that matters is the first-hour rating.
How tank sizing actually works
The first-hour rating (FHR) is how many gallons of hot water a tank can deliver in one hour, starting full. It depends on the tank's capacity and on how fast the burner or element reheats water. So a gas tank and an electric tank with the same gallons on the label can have very different first-hour ratings.
You'll find the FHR on the yellow EnergyGuide label, listed as "Capacity (first hour rating)" in the top left corner, and in the manufacturer's literature.
The U.S. Department of Energy's method takes three steps:
- Pick your busiest hour of the day for hot water.
- Add up the hot water used in that hour, with the figures below.
- Choose a heater with a first-hour rating at least that high.
DOE's worksheet figures per use:
| Activity | Hot water per use |
|---|---|
| Shower | 20 gallons |
| Shaving | 2 gallons |
| Hand dishwashing or food prep | 3 gallons |
| Automatic dishwasher | 7 gallons |
| Clothes washer, top-loader | 25 gallons |
| Clothes washer, front-loader | 15 gallons |
DOE's own example: three showers, one shave and one round of hand dishwashing in the same hour come to 66 gallons, so that home needs a first-hour rating of 66 gallons or more. Rheem runs the same math with two showers, a laundry load and some kitchen use and lands at 55 gallons. The two use slightly different per-shower figures. Use DOE's if you want a margin, especially with older shower heads.
Rheem also flags the homes that tend to need more capacity: three or more bathrooms, big soaking tubs, high-demand appliances, or a household that's about to grow. A deep tub can empty a small tank on its own.
Tankless sizing
A tankless heater has no stored water, so the first-hour rating doesn't apply. Two numbers do: flow rate and temperature rise.
Flow rate. List the fixtures you run at the same time and add up their gallons per minute. DOE's example is a 0.75 gallon-per-minute faucet plus a 2.5 gallon-per-minute shower head, which needs at least 3.25 gallons per minute. Rheem's rough fixture figures are about 2 gallons per minute for a shower, 0.5 for a bathroom faucet, 1 to 2 for a kitchen sink and 1 to 1.5 for a washing machine. If you don't know a fixture's flow, time how long it takes to fill a bucket.
Temperature rise. Subtract the incoming water temperature from the temperature you want. DOE says to assume 50°F incoming unless you know better, and to aim for 120°F in most homes, which is a 70°F rise. Cold-climate homes in winter see colder water and need more rise.
Then compare with the unit's rating. DOE says gas tankless units typically manage a 70°F rise at about 5 gallons per minute, and electric units at about 2. That's why electric tankless struggles with two showers at once in a cold climate. Our tankless water heater sizing guide goes deeper.
Heat pump water heaters
Heat pump tanks are sized the same way as other tanks, by first-hour rating. The catch is that a heat pump heats more slowly than a gas burner, so its FHR can be lower than the gallons suggest. Read the EnergyGuide label rather than going by tank size. See heat pump water heaters.
What to do next
- Add up your busiest hour with the DOE figures, then look for a tank whose first-hour rating meets or beats it.
- If you keep running out of hot water today, check that the current heater is working properly before upsizing. A failed element or a tank full of sediment cuts what it can deliver. Start with water heater runs out of hot water.
- Deciding on fuel? See gas vs electric water heaters and tankless vs tank.
- Pricing it out? Bigger tanks cost more to buy and install. See water heater installation cost.
FAQ
What size hot water tank do I need for a family of four?
Rheem's rough guide puts 2 to 4 people at 40 to 50 gallons and 4 to 5 people at 50 to 55. Check it against your busiest hour with DOE's worksheet. Four people showering back to back in the morning need a higher first-hour rating than the head count suggests.
What size on-demand water heater do I need?
Add up the gallons per minute of every fixture you'd run at once, then work out the temperature rise from your incoming water to about 120°F. Buy a unit rated for that flow at that rise. In cold climates, size for winter water temperatures.
How big a tankless water heater do I need for a family of five?
It depends on how many fixtures run together, not on head count. Two showers and a kitchen sink at once can need well over 5 gallons per minute by Rheem's fixture figures, which is beyond what DOE says a typical electric unit delivers at a 70°F rise. Larger families often end up with a gas unit or two units.
What size wire does a water heater need?
It depends on the heater's wattage and voltage, which are on the data plate. The installation manual gives the required breaker and wire size. Have a licensed electrician confirm the circuit, especially when going to a larger tank or an electric tankless unit.
Sources
- U.S. Department of EnergySizing a New Water Heater (opens in a new tab)
- RheemWhat Size Gas Water Heater Do I Need for My Home? (opens in a new tab)
- RheemIs a Tankless Electric Water Heater Right for Your Home? (opens in a new tab)