How this calculator works
The U.S. Department of Energy sizes a storage water heater by the hot water used in the household's busiest hour, called the peak hour demand. A shower uses about 10 gallons of hot water, a bath 10, a dishwasher cycle 6, a warm-wash load of laundry 7, a shave 2, and a hand wash or a food preparation use about 1. Add them up for the morning rush. The water heater's first hour rating (FHR) is the hot water it can deliver in one hour starting full, and it should match that peak hour demand. A tank gives about 70% of its size at hot temperature, and the burner or element recovers about 40 gallons an hour for gas and 20 for electric. We choose the smallest standard tank that meets the demand, and not less than the usual size for the number of people.
Worked example: two showers, two shaves, four sink uses, a dishwasher cycle and a load of laundry use 41 gallons of hot water in the busiest hour. A gas tank meets that at 30 gallons, since 30 gal gives about 61 gal in the first hour, but a household of four usually wants 40 gallons, so the answer is 40 gallons (an estimated FHR of 68). An electric tank for the same hour needs 30 gallons by the demand and 40 after the household rule. A busier household of five with three showers uses 55 gallons and needs a 50 gallon gas tank, while two people with one shower use 14 gallons and need a 30 gallon tank.
Tank size for common households (gas and electric)
The smallest standard tank that meets the first hour rating, and not smaller than the usual size for the number of people.
| Household and busiest hour | Hot water in the hour | Gas tank | Electric tank |
|---|---|---|---|
| Two people: one shower, a shave, a dishwasher | 20 gal | 30 gal | 30 gal |
| Three people: two showers, a dishwasher | 31 gal | 40 gal | 40 gal |
| Four people: two showers, dishwasher, laundry | 41 gal | 40 gal | 40 gal |
| Four people: three showers, dishwasher, laundry | 51 gal | 40 gal | 50 gal |
| Five people: three showers, a bath, dishwasher, laundry | 65 gal | 50 gal | 65 gal |
Tips before you buy
- Size the water heater on the busiest hour, not on the number of bedrooms. The morning rush is the test: showers back to back with the dishwasher running.
- The first hour rating on the label is a better guide than the tank size, because two 50-gallon heaters can have very different ratings.
- Electric heaters recover slowly, so a household that uses a lot of hot water in one hour needs a bigger electric tank than a gas one.
- Low-flow showerheads (2.0 gpm or less) cut the peak hour demand, and can let you choose a smaller tank.
- Water heaters last 10 to 15 years. Before you buy, check the space, the venting, the gas or power supply and the local code.
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Frequently asked questions
What size water heater do I need for a family of 4?
Usually a 40 to 50 gallon tank. In the busiest hour a family of four uses about 41 gallons with two showers, a dishwasher and a load of laundry, which a 30 gallon gas heater delivers; the common rule for four people is 40 gallons or more.
What is a first hour rating?
The number of gallons of hot water a storage water heater can deliver in one hour starting from full. It is on the EnergyGuide label, and should be about the same as your peak hour demand.
How many gallons of hot water does a shower use?
About 10 gallons of hot water for a typical shower (the Department of Energy figure). A bath is about 10, a dishwasher cycle 6 and a load of laundry on warm 7.
Do electric water heaters need to be bigger than gas?
Often yes. They recover slowly (about 20 gallons an hour against 40 for gas), so for the same busiest hour an electric tank needs to hold more: 30 gallons here, against 30 for gas.
Is a 50 gallon water heater big enough?
For most families of three to four, yes. A 50 gallon gas heater has a first hour rating of about 75 gallons, and an electric one about 55, against a busiest hour of 41 to 55 gallons.
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