What the first-hour rating measures
The first-hour rating is the amount of hot water a heater can supply in one hour, starting with a full tank of hot water. It depends on the tank volume and on how fast the burner or element can reheat what is drawn off. That is why a 40-gallon gas tank with a strong burner can outperform a 50-gallon electric tank, and why DOE tells buyers to compare the rating rather than the volume.
The peak-hour worksheet
List what your household does with hot water in its busiest hour, usually a weekday morning. Multiply each activity by the gallons in the table and by the number of times it happens in that hour.
| Activity | Hot water per use |
|---|---|
| Shower | 15 gal |
| Bath | 20 gal |
| Clothes washing | 32 gal |
| Automatic dishwasher | 14 gal |
| Food preparation | 5 gal |
| Shampooing hair | 4 gal |
| Shaving | 2 gal |
| Washing hands and face | 2 gal |
| Hand dishwashing | 2 gal |
Source: U.S. Department of Energy, Technology Fact Sheet "Water Heating," August 2001 (chart credited to the Gas Appliance Manufacturers Association). The chart assumes no water-saving fixtures.
Worked examples
The DOE fact sheet's own example, two showers, two shampoos, a shave, two hand washes and some food preparation, comes to 49 gallons. The rows below use the same per-use figures for other households. They are arithmetic from the worksheet, not recommendations for a particular model.
| Busiest hour | Peak-hour demand |
|---|---|
| 2 people: two showers, one shave, two hand washes, food prep | 41 gal |
| DOE example: two showers, two shampoos, one shave, two hand washes, food prep | 49 gal |
| 4 people: three showers, two shampoos, one shave, two hand washes, food prep | 64 gal |
| Same four, plus a dishwasher run | 78 gal |
| Five people: four showers, one bath, three shampoos, one shave | 94 gal |
A laundry load in the same hour adds 32 gallons, which is the easiest demand to move. Running the washer at another time often saves a tank size.
Adjusting for water-saving fixtures
The worksheet dates from a time of 2.5-gallon-per-minute showerheads. EPA WaterSense showerheads are limited to 2.0 gallons per minute, and WaterSense bathroom faucets to 1.5 against a standard 2.2. A household with those fixtures uses less hot water per shower than the table assumes, so its real peak is lower than the worksheet total. It is fair to trim the shower figure, but keep the worksheet total as the ceiling.
Sizing a tankless unit instead
For tankless models, count the fixtures that run at the same time and add their flow rates. DOE's building science guidance puts high-efficiency whole-house units at about 2 to 5 gallons per minute. Two WaterSense showers and one bathroom faucet add up to 5.5 gallons per minute, beyond one typical unit. The deciding factor is the temperature rise: colder incoming water means fewer gallons per minute at 120°F. Our tankless installation guide works through it.
Common sizing mistakes
- Matching the old tank. The household may have changed since the last heater went in, in either direction.
- Comparing volume, not first-hour rating. Two 50-gallon tanks can differ by 20 gallons or more in the first hour.
- Forgetting recovery on heat pump models. In heat pump mode they reheat slowly, so installers often go one size up. See heat pump water heater requirements.
- Oversizing for safety. A tank much larger than the peak hour loses more heat standing idle, every day, for its whole life.
When a size change affects the price
Moving up a size can mean a taller or wider tank that no longer fits the closet or the old pan, a larger gas connector, or a bigger vent. Moving down rarely costs extra. Ask for the replacement quote at the size you worked out here, and compare it with the like-for-like figure on our replacement cost page. If the current heater simply ran out of hot water one day, rule out a failed element or dip tube first; the no-hot-water checklist covers both.
Sizing questions, answered with the worksheet
What size water heater do I need for a family of 4?
Work out the busiest hour rather than counting people. Using the Department of Energy per-use figures, three showers, two shampoos, one shave, two hand washes and food prep add up to about 64 gallons; add a dishwasher run in the same hour and it reaches about 78. Look for a first-hour rating within a couple of gallons of your own total.
Is a bigger tank always safer?
No. The same DOE fact sheet warns that an oversized unit carries a higher purchase price and higher energy costs, because a larger tank loses more heat while it sits. Size to the busiest hour, not to the worst hour you can imagine.
Does a large soaking or walk-in tub change the size?
Often, yes. The DOE worksheet assumes 20 gallons of hot water for a bath, and a large soaking tub can take far more. Check the tub’s capacity on its specification sheet and count it as one use in your busiest hour.
Where do I find a heater’s first-hour rating?
On the manufacturer’s specification sheet for the model, and usually in the product listing. Two tanks of the same volume can have very different first-hour ratings because burner and element sizes differ.
Sources
- Technology Fact Sheet: Water Heating (DOE/GO-102001-0785), first-hour rating worksheet, U.S. Department of Energy, August 2001.
- Water Heaters, Gas-Fired Tankless (Building Science Education), U.S. Department of Energy.
- WaterSense: Showerheads, U.S. Environmental Protection Agency, updated March 3, 2026.
- WaterSense: Bathroom Sink Faucets, U.S. Environmental Protection Agency, updated June 16, 2026.
Reviewed September 27, 2026. Found an error? Write to admin@plumbingrepairquotes.com.