Water heater size calculator: first hour rating

Updated October 28, 2026

Tank size alone does not tell you if you will run out of hot water. The number to match is the first hour rating. Count what your household does in its busiest hour and the calculator adds it up.

Your busiest hour

Count how many times each happens in the single busiest hour of a normal day, often the morning rush.

Showers 10 or 20 gal each

Baths 20 gal each

Shaving at the sink 2 gal each

Hand dishwashing or food prep 4 gal each

Dishwasher loads 6 or 14 gal each

Clothes washer loads 7 or 32 gal each

Gallons per use

How the calculator works

The U.S. Department of Energy guide Sizing a New Water Heater says to size a tank water heater by its first hour rating: the gallons of hot water it can supply in an hour, starting with a full tank. You estimate your peak hour demand, then look for a model whose first hour rating matches it within 1 or 2 gallons.

Peak hour demand = the sum of (times used in the busiest hour × gallons of hot water per use).

Use	DOE gallons per use	AHRI gallons per use
Showers	10	20
Baths	20	20
Shaving at the sink	2	2
Hand dishwashing or food prep	4	4
Dishwasher loads	6	14
Clothes washer loads	7	32

The DOE figures assume water-saving fixtures. The AHRI peak hour worksheet assumes no water conservation measures, so its numbers for showers, dishwashers and clothes washers are higher. The DOE worksheet does not list baths, so both columns use AHRI's 20 gallons. If your showerheads and appliances are older, use the AHRI column.

Worked example

This is the DOE guide's own example: 3 showers (30 gallons), 1 shave (2 gallons) and 1 hand dishwashing (4 gallons) in the same hour add up to a peak hour demand of 36 gallons. DOE suggests a heater with a first hour rating of 34 to 38 gallons.

Tank size and first hour rating are different numbers

Two 50-gallon tanks can have different first hour ratings, because the rating also depends on the burner or element size and the fuel. Gas heaters usually recover faster than electric ones of the same size. Compare first hour ratings on the EnergyGuide label, not tank gallons.

If your showers already run cold partway through, the cause may be a failed part rather than an undersized tank. Our guide on why a shower turns cold after 5 minutes walks through the checks. For a tankless heater, use the tankless water heater size calculator instead, because tankless units are sized by flow rate.

Frequently asked questions

What size water heater do I need for a family of four?

It depends on what happens in your busiest hour, not on headcount. Four people showering back to back at 10 gallons each, plus some dishwashing, needs a first hour rating in the mid 40s of gallons on the DOE figures. With older fixtures at AHRI's 20 gallons per shower, it is closer to 85.

What is a first hour rating?

The number of gallons of hot water a tank heater can supply in one hour, starting with a full tank. It depends on tank size, the heat source and the burner or element size, and it is listed on the EnergyGuide label.

Is a bigger tank always better?

No. DOE suggests matching the first hour rating to your peak hour demand within 1 or 2 gallons. A tank far larger than you need keeps extra water hot all day, which adds standby energy use.

Does this work for tankless water heaters?

No. Tankless heaters have no first hour rating. They are sized by the gallons per minute you need at once and the temperature rise. Use our tankless calculator for those.

Sources

Sizing a New Water Heater, U.S. Department of Energy (PDF copy)
Worksheet for Estimating Peak Hour Demand / First Hour Rating, AHRI