Key takeaways
- Compare a heater's first-hour rating with your peak-hour demand, rather than tank size or family size.
- Choose a first-hour rating equal to or greater than your demand.
- If you've been running out of hot water, don't assume your old unit was the right size.
Count how much hot water you use in your single busiest hour, usually in the morning. The tool adds it up and gives you a number to compare on the EnergyGuide label.
How the calculator works
Peak-hour demand = gallons per use × uses in your busiest hour, added together
PNNL's Building America guidance says to choose a storage heater with a first-hour rating equal to or greater than that demand, using high-end usage values so the unit keeps serving you over its life.
| Use | Gallons per use |
|---|---|
| Shower | 10 |
| Shaving | 2 |
| Hand dishwashing or food prep | 4 |
| Automatic dishwasher | 6 |
| Clothes washer | 7 |
Where these come from: a plumbing company's worksheet. The Department of Energy's sizing page wouldn't open when we tried it on November 2, 2026, so treat them as planning figures until we can confirm them there.
Worked example
Three showers, one shave, one hand-washing session and one laundry load in the same hour:
- (3 × 10) + (1 × 2) + (1 × 4) + (1 × 7) = 43 gallons
- So look for a first-hour rating of 43 or more
What's a first-hour rating?
It's the number of gallons of hot water a heater can deliver in one hour, starting from a fully heated tank. It's a better comparison than tank size, because two tanks of the same size can differ. PNNL says the test conditions are 67.5°F air and 58°F supply water, so colder conditions can lower real output.
Heat pump water heaters
Ratings work differently here. PNNL's guide says units without electric-resistance backup have lower first-hour ratings, so compare with care, and it suggests a larger tank in colder climates. The unit also needs enough air:
| Room volume | PNNL's guidance |
|---|---|
| 700 cu ft or more | Sufficient |
| 450 to 700 cu ft, inside the thermal envelope | Will likely run efficiently most of the time |
| Below 450 cu ft | Ventilate the room |
Volume is length × width × height. Follow the maker's instructions for your model.
Troubleshooting
- You run out of hot water: PNNL says not to assume the old heater was sized correctly, so choose a higher first-hour rating and consider your busiest hour again.
- Your household changed: run the worksheet again.
- A rule says "family of four = X gallons": treat it as a rule of thumb, because habits vary and tank size isn't the rating.
What to leave to a professional
We don't cover gas lines, venting, combustion air or temperature settings here. The 2021 IRC has rules on garage ignition sources, relief valve discharge and seismic strapping, and it restricts some gas appliances in bedrooms and bathrooms. Replacing one can need a permit, so ask your local building department.
Frequently asked questions
What size water heater for a family of four?
It depends on your busiest hour rather than the headcount. Run the worksheet, using how your family actually uses water.
Where do I find the first-hour rating?
It's usually on the EnergyGuide label, or in the product specifications.
Why use high-end usage values?
PNNL suggests it so the unit still serves you over its life, which it puts at about 13 years on average.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- PNNL Building America: First Hour Rating tool and heat pump water heater guide
- 2021 International Residential Code, Chapter 28 and Section G2406.2
- Benjamin Franklin Plumbing: Tips for choosing a water heater (secondary source for usage values)