A storage water heater is sized by its first-hour rating (FHR): how much hot water it can deliver in the busiest hour of your day. Enter who lives in the house and what happens during that peak hour to get the FHR you need and a tank size for gas, electric or heat pump models.
Last updated:
Household
Half baths count as 0.5.
°F
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Busiest hour: uses in that hour
Hot water per use (gallons)
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Enter your values to see the result.
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How to use the Water Heater Size Calculator
Enter the number of people and bathrooms in the home.
Think about your busiest hour (often weekday mornings) and enter how many showers, baths and other hot water uses happen in it.
Adjust gallons per use if your showers are longer or your fixtures are low-flow.
Choose the fuel type and enter your incoming water temperature and tank setting.
Read the first-hour rating you need and the recommended tank size, then compare it with the FHR printed on the heater's EnergyGuide label.
How tank water heaters are sized
The U.S. Department of Energy recommends sizing a storage water heater by its first-hour rating (FHR), not by tank volume alone:
Peak-hour demand = sum of (uses in your busiest hour × gallons of hot water per use).
Choose a heater with an FHR within 1 or 2 gallons of, or above, the peak-hour demand.
The FHR depends on how much hot water the tank can draw before it gets too cool (roughly 70% of the tank) plus the recovery rate, the gallons the burner or element can heat in one hour:
Example: a 40,000 BTU/h gas burner at 78% efficiency with a 70 °F rise recovers 40,000 × 0.78 ÷ (8.33 × 70) ≈ 53.5 gal/h. A 4,500 W electric element (15,355 BTU/h) recovers only about 25.8 gal/h at the same rise, which is why electric tanks are usually one size larger. Because cold water mixes into the tank while it is being drawn, this calculator uses a conservative 0.68 effective efficiency for gas recovery in the first hour, which lands close to the published FHR of typical tanks (about 55–60 gallons for 30-gallon gas, 65–75 for 40-gallon gas and 80–85 for 50-gallon gas).
Typical hot water per use
Use
Hot water (gal)
Liters
Shower
10
38
Bath
20
76
Shaving
2
8
Hand & face washing
4
15
Hand dishwashing
4
15
Automatic dishwasher
6
23
Food preparation
5
19
Clothes washer (warm/hot)
7
26
These are typical estimates used in sizing worksheets. Long showers, large soaking tubs and older fixtures use more.
Rule-of-thumb tank sizes by household
Household
Gas tank
Electric / heat pump tank
1–2 people
30 gal
40 gal
3 people
40 gal
50 gal
4 people
40 gal
65 gal
5 people
50 gal
65 gal
6+ people
75 gal
80 gal
Homes with three or more bathrooms in use at the same time should step up one size. For very high or continuous demand, look at tankless heaters or two tanks.
Frequently asked questions
How do I choose the right water heater size?
Estimate your peak-hour demand: the gallons of hot water used in the busiest hour of the day. Then pick a heater whose first-hour rating (FHR) is equal to or slightly higher than that number. The U.S. Department of Energy recommends this method because two heaters with the same tank size can have very different FHRs.
What is a first-hour rating?
The first-hour rating is the number of gallons of hot water a tank heater can supply in one hour starting with a full tank. It combines the usable stored water with the water the burner or elements can heat during that hour. It is printed on the yellow EnergyGuide label and in the spec sheet.
What size water heater for a family of 4?
A family of four with two bathrooms usually needs a first-hour rating of about 60 to 80 gallons, which typically means a 40 to 50 gallon gas tank or a 50 to 65 gallon electric or heat pump tank. If several showers happen back-to-back in the morning, use the peak-hour calculation instead of the household rule of thumb.
Why does an electric water heater need a bigger tank than gas?
Electric resistance elements (usually 4,500 W) heat water more slowly than a 36,000 to 40,000 BTU/h gas burner, so they recover only about 20 gallons per hour versus roughly 40 for gas at a 90 °F rise. To deliver the same first-hour rating, an electric heater needs more stored water.
How much hot water does a shower use?
About 2 gallons per minute of mixed water, of which roughly 60% to 70% is hot. A typical 8 minute shower uses 10 to 12 gallons of hot water, which is the value used here by default. Longer showers or older 2.5 GPM heads use more.
Does the incoming water temperature matter?
Yes. Colder incoming water means a bigger temperature rise, so the heater recovers fewer gallons per hour and the first-hour rating drops. The tank size table on this page adjusts recovery for your inlet and tank temperatures.
Is my data stored?
No. All calculations run in your browser, and your inputs are only remembered locally on your device.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.