Water heater size calculator
A tank water heater is sized by its first-hour rating: the gallons of hot water it can deliver in an hour starting with a full tank. Count what your household uses in its busiest hour and the calculator gives the rating to look for.
- Peak-hour demand65 gallons
- First-hour rating to look for65 to 67 gal
Peak-hour demand: 3 × 20 + 1 × 2 + 1 × 3 = 65 gallons.
Choose a tank water heater with a first-hour rating at or just above that figure. The rating is printed on the yellow EnergyGuide label and in the product specifications. It is not the same as the tank capacity.
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Hot water use by activity
The US Department of Energy (DOE) sizes a tank water heater by its first-hour rating, not by tank volume. The first-hour rating is the gallons of hot water the heater can deliver in an hour, starting with a full tank. This table gives DOE’s hot water use for each activity.
| Activity | Hot water used (gallons each) |
|---|---|
| Showers | 20 |
| Shaves | 2 |
| Hand dishwashing or food prep | 3 |
| Dishwasher loads | 7 |
| Washer loads, top-loader | 25 |
| Washer loads, front-loader | 15 |
These are DOE averages. Older copies of the worksheet that circulate online use lower figures for several activities. This table follows DOE’s page as archived on December 30, 2024.
Peak-hour demand for four households
The peak-hour demand is the hot water your household uses in its busiest hour. These four households are examples. We chose the counts, so they are not DOE figures.
| Household | Peak-hour demand (gal) | Rating to look for (gal) |
|---|---|---|
| One person: 1 shower, 1 shave | 22 | 22 to 24 |
| Two people: 2 showers, 1 shave, 1 round of hand dishwashing | 45 | 45 to 47 |
| Family of four, top-loader: 3 showers, 1 shave, 1 dishwasher load, 1 top-loader washer load | 94 | 94 to 96 |
| Family of four, front-loader: 3 showers, 1 shave, 1 dishwasher load, 1 front-loader washer load | 84 | 84 to 86 |
DOE says to look for a first-hour rating within 1 or 2 gallons of the peak-hour demand. The calculator shows the demand and the demand plus 2 gallons.
How the calculator works
The calculator follows DOE’s method. Pick the hour of the day when your household uses the most hot water. Multiply the gallons for each activity by how many times it happens in that hour. Add the results. That total is the peak-hour demand.
The first-hour rating is printed on the EnergyGuide label as “Capacity (first hour rating)”. It is not the tank capacity. The gallons per activity are DOE figures. DOE has removed the page from energy.gov, so the source link below goes to an archived copy.
Limits of this calculator
- The gallons are averages. Your showers may use more or less than 20 gallons.
- It sizes for one hour. The method does not estimate how much hot water the household uses in a day.
- Some uses are not on the list. A bathtub fill, for example, is not counted.
- It covers tank water heaters. Size a tankless unit by flow rate instead.
Sources
- US Department of Energy, Energy Saver, Sizing a New Water Heater (archived copy of December 30, 2024): https://web.archive.org/web/20241230224527/https://www.energy.gov/energysaver/sizing-new-water-heater
Related
For a tankless unit, use the tankless water heater size calculator. Read how long a tank lasts on the water heater lifespan page, or date one with the water heater age lookup.
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Calculator last reviewed on October 5, 2026. Method and sources.