What Size Water Heater Do I Need? Tank Size and First Hour Rating
Part of the project planner: Planning pools, ponds and water storage.
A family of four usually wants a 40 to 50 gallon water heater, but the best way to size one is by the hot water you use in your busiest hour. A morning with two showers, two shaves, a dishwasher cycle and a load of laundry uses about 41 gallons, and the water heater's first hour rating (FHR) should match that. The water heater size calculator adds up your own busiest hour and finds the tank, for gas or electric.
Hot water used per task
The U.S. Department of Energy's estimates of hot water per use (the rest of the water is cold):
| Use | Hot water |
|---|---|
| Shower | 10 gal |
| Bath | 10 gal |
| Dishwasher cycle | 6 gal |
| Clothes washer (warm) | 7 gal |
| Shave | 2 gal |
| Hand and face washing, food preparation | 1 gal |
Add up what could happen in one hour, which is usually the morning rush, and you have the peak hour demand.
Tank size for common households
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 |
| Six people: four showers, dishwasher, two loads of laundry | 72 gal | 50 gal | 80 gal |
First hour rating by tank size
A tank does not deliver its size and no more: it is heated as you draw from it. A rule of thumb is about 70% of the tank plus the recovery for the hour (about 40 gallons for gas, 20 for electric):
| Tank size | Gas first hour rating | Electric first hour rating |
|---|---|---|
| 30 gal | 61 gal | 41 gal |
| 40 gal | 68 gal | 48 gal |
| 50 gal | 75 gal | 55 gal |
| 65 gal | 86 gal | 66 gal |
| 80 gal | 96 gal | 76 gal |
Always check the first hour rating on the label of the model you are buying. Two 50-gallon heaters can have very different ratings, and a heat pump (hybrid) heater recovers slowly in its efficient mode.
Choosing between a tank and a tankless heater
- Tank: costs less to buy and install, and delivers a large burst of hot water. It loses a little heat while it stands.
- Tankless: heats on demand, lasts longer and saves space, but it needs a large gas line or heavy electric supply, and is sized by flow (gallons per minute) and temperature rise. Use the tankless water heater calculator.
- Heat pump (hybrid): uses much less electricity, but recovers more slowly and needs space and air around it.
Before you buy
- Check the space, the venting, the gas or electric supply, and the local permit rules. A bigger tank may not fit the closet.
- A low-flow showerhead (2.0 gallons a minute or less) cuts the hot water used, and may let you choose a smaller heater.
- Insulate the first few feet of pipe, and set the thermostat to 120°F for safety and savings.
- Water heaters last about 10 to 15 years. A leak is the sign to replace it.
For running costs, see the appliance electricity cost calculator and the heating cost comparison. This is a sizing estimate: ask a plumber or an installer to confirm the size and the code requirements for your home.
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 of hot water with two showers, a dishwasher cycle and a load of laundry, which a gas heater of 30 gallons delivers; the usual rule for four people is 40 gallons or more.
What is the first hour rating of a water heater?
The gallons of hot water the heater can deliver in one hour, starting full. It is shown on the yellow EnergyGuide label, and should match the hot water you use in your busiest hour.
Is a 40 gallon or a 50 gallon water heater better?
A 40 gallon gas heater has a first hour rating of about 68 gallons and an electric one about 48, which covers a busiest hour of 41 gallons. Go to 50 gallons if you have a large tub, a big family or several showers back to back.
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 warm-water load of laundry 7.
Do I need a bigger tank if it is electric?
Often. Electric heaters recover slowly (about 20 gallons an hour against 40 for gas), so a busier hour needs a bigger electric tank. For a busiest hour of 51 gallons (three showers, a dishwasher and a load of laundry) a gas tank of 30 gallons meets the first hour rating, but an electric tank needs 50 gallons.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.