Quick answer
Choose a tank water heater with a first-hour rating at least as high as your peak-hour demand: the hot water your household uses in its busiest hour. AHRI's worksheet counts about 20 gallons per shower, 4 for a shampoo or hand dishwashing and 2 for a shave, so 3 showers, a shave, a shampoo and hand dishwashing need 70 gallons. The first-hour rating is on the yellow EnergyGuide label.
Tank size vs first-hour rating
Federal rules define a water heater's first-hour rating as an estimate of the most hot water it can supply in an hour that starts with the tank fully heated, and ENERGY STAR notes it is measured at a 125°F outlet. It combines the stored water with how fast the heater recovers, so two 50 gallon tanks can have different ratings. The FTC requires the EnergyGuide label on a tank heater to show both its storage volume and its first-hour rating.
Step 1: find your busiest hour
Pick the hour of the day when your household uses the most hot water, usually the morning. Count each use, and multiply by AHRI's gallons of hot water per use:
| Use | Gallons each time |
|---|---|
| Showers | 20 |
| Baths | 20 |
| Shaving | 2 |
| Hand and face washing | 4 |
| Hair shampoo | 4 |
| Hand dishwashing | 4 |
| Automatic dishwasher | 14 |
| Food preparation | 5 |
| Automatic clothes washer | 32 |
AHRI's figures assume no water-saving fixtures. DOE's Building America tool recommends sizing with full figures like these rather than low-flow ones, so the heater keeps up through its life, which it puts at 13 years on average.
Step 2: choose the first-hour rating
DOE's Building America Solution Center says to select a storage water heater with a first-hour rating equal to or greater than the peak-hour demand. Using AHRI's own example, 3 showers, a shave, a shampoo and hand dishwashing add up to 70 gallons, so look for a first-hour rating of 70 gallons or more. The water heater size calculator adds up your own busiest hour.
Federal test rules sort heaters into draw patterns by first-hour rating: under 18 gallons is very small, 18 to under 51 is low, 51 to under 75 is medium, and 75 or more is high. A tank rated at 70 gallons falls in the medium group, and its EnergyGuide label compares it only with other medium models.
How much hot water a home uses in a day
DOE's Building America strategy guideline cites field data averaging about 50.6 gallons of hot water a day for a single-family home, or 17 gallons per person, and groups homes into low, moderate, above-average and high use at roughly 20, 45, 65 and 110 gallons a day. Daily use matters for operating cost; the peak hour is what decides the size.
Heat pump water heaters
ENERGY STAR's guidance for builders says to upsize a heat pump water heater's tank over standard practice, so its less efficient backup elements run less. Its NextGen program sets minimum tanks, for example 59 gallons for a home with 2 to 2.5 bathrooms and 3 bedrooms, and it suggests upsizing further for households with teenagers or a taste for baths. The same table lists the Uniform Plumbing Code's minimum first-hour rating for that home as 62 gallons. ENERGY STAR also says to follow the local plumbing code and the manufacturer's sizing.
Set it to 120°F
DOE and the Consumer Product Safety Commission recommend 120°F. CPSC warns that water at 140°F can cause third-degree burns in six seconds, and that young children and older adults are most at risk.
Thinking about going tankless? See tank vs tankless sizing.
Frequently asked questions
What size water heater for a family of 4?
It depends on how many people shower in the same hour. Using AHRI's figures, four showers back to back use about 80 gallons, so a family that showers that way needs a tank with a first-hour rating of 80 gallons or more. Spread the showers out and a smaller rating works.
Is a 40 or 50 gallon water heater enough?
Check the first-hour rating on the EnergyGuide label rather than the tank size, and compare it with your busiest hour of hot water use. Two tanks of the same size can have different first-hour ratings.
Where do I find the first-hour rating?
On the yellow EnergyGuide label and in the specifications. The FTC requires tank water heater labels to show both the storage volume and the first-hour rating.
What is peak-hour demand?
The hot water your household uses in its busiest hour of the day, which AHRI's worksheet adds up from the number of showers, baths, sink uses and appliance loads in that hour.
Calculators for this
Water Heater Size
Find what size water heater you need: the first-hour rating for a tank from your busiest hour of hot water use, or the flow and temperature rise for a tankless.
Heating Cost Comparison
Compare the cost of heat from natural gas, propane, heating oil, electric heat and a heat pump, per million Btu and for a whole winter, with your own prices.
Space Heater Size
What size space heater do you need?
Published 2026-11-07. Sources: AHRI, Worksheet for Estimating Peak Hour Demand/First Hour Rating; DOE Building America Solution Center, DHW First Hour Rating tool; DOE Building America, Strategy Guideline: Proper Water Heater Selection (2015); 10 CFR 430 Subpart B, Appendix E (water heater test procedure); 16 CFR Part 305 (EnergyGuide labels); ENERGY STAR, Residential water heaters key product criteria; ENERGY STAR, Heat pump water heater design considerations; CPSC, Avoiding Tap Water Scalds. See how we calculate.