Water Heater Size Calculator

How to work out your busiest hour

You are not measuring anything with a tape this time. Think through the worst hour on a normal day. For most households that is the weekday morning: who showers, who shaves, and is someone rinsing breakfast dishes or starting the washer while it happens? Only count things that fall inside the same hour.

Be honest about whether showers truly overlap. If one person showers at 6:30 and the rest at 8:00, those are different hours, and the first hour rating only has to deal with one of them. Teenagers who take long showers, on the other hand, are a good reason to raise the gallons per shower in the further options.

DOE's example: a peak hour demand of 36 gallons. The band is the first hour rating to look for.

Then go to the store and read the EnergyGuide label, which is the yellow sticker on the heater. Look for capacity, listed as first hour rating. It is often not the same as the number of gallons on the tank. DOE notes that the label rule does not cover heat pump water heaters, so for those use the maker's literature.

A worked example

This is DOE's own example, from its worksheet as archived in 2017: three showers, a shave and some hand dishwashing in one hour. These lines come from the same formula the calculator above runs.

DOE's worksheet example

Three showers: 3 × 10 = 30 gal
A shave and hand dishwashing: 2 + 4 = 6 gal
Peak hour demand: 36 gal
First hour rating to look for: 34 to 38 gal

DOE prints the same 36 and the same range of 34 to 38. There is a caveat. A search result suggests that a later DOE page may use 20 gallons per shower, though we could not read it. At that figure this household's demand would be 66 gallons, which would be a different heater. If showers are long at your house, use the higher figure. To see how much hot water you use over a day, try the water usage calculator, and the electricity cost calculator estimates running an electric heater.

Peak hour demand for common households

These are examples, not recommendations, because your hour is your own. Find the row nearest yours, then count your real morning. Rheem's table is a useful cross-check: 30 to 40 gallons for 1 to 2 people, 40 to 50 for 2 to 4, 50 to 55 for 4 to 5, and 75 or tankless beyond that.

Worked out by this calculator at DOE's figures.
Busiest hour	Peak demand	First hour rating
One person, one shower and a shave	12 gal	10 to 14 gal
Couple, two showers and breakfast dishes	24 gal	22 to 26 gal
Family of 3, three showers	30 gal	28 to 32 gal
Family of 4, three showers and a wash	39 gal	37 to 41 gal
Family of 4, four showers and the dishwasher	46 gal	44 to 48 gal
Family of 5, four showers, wash and dishes	51 gal	49 to 53 gal

Mistakes that buy the wrong heater

Choosing by family size. Two houses with four people can have very different mornings.
Reading the tank size as the answer. A 40 gallon tank with fast recovery can beat a 50 gallon one with slow recovery. The first hour rating covers both.
Using total daily use. DOE warns that the worksheet does not estimate it, and a water heater is sized for the peak.
Counting things that do not overlap. Laundry at night does not belong in your morning hour.
Assuming combination systems work the same. Space and water heating together is a job for a qualified contractor, as DOE says.

Questions

What size water heater do I need for a family of 4?

A family of 4 usually needs a tank of about 40 to 55 gallons, by Rheem's table, but your busiest hour decides. Three showers, a shave and some washing up comes to 36 gallons, so you would look for a first hour rating of 34 to 38.

What is first hour rating?

First hour rating is the number of gallons of hot water a heater can supply per hour, starting with a full tank, as DOE defines it. It is on the EnergyGuide label as capacity. It tells you more than tank size, because it includes how fast the heater recovers.

How many people is a 50 gallon water heater for?

A 50 gallon heater typically suits 2 to 5 people, as Rheem's table has 40 to 50 gallons for 2 to 4 people and 50 to 55 for 4 to 5. Two heaters of the same size can have different first hour ratings, so check that number.

Do I need a 40 or a 50 gallon water heater?

Choose by first hour rating, not gallons. Work out your busiest hour and buy the one whose rating is within a gallon or two of it. Many households of 2 to 4 are fine with 40 gallons unless showers overlap heavily.

How much hot water does a shower use?

DOE's worksheet, as archived in 2017, says 10 gallons of hot water per shower. A later version may use a larger figure, which we could not confirm, and your head and habits count. Change the figure in the further options if you know yours.

Why is there no bath on the list?

DOE's worksheet has no row for a bath or a sink, and we will not invent gallons for them. If a bath is part of your busiest hour, measure how much hot water yours takes and add it to the total yourself.

This water heater size is a rule of thumb from DOE's peak hour worksheet, which we could read only as a 2017 archive. Showerheads, lingering teenagers and overlapping mornings all change it, so you can adjust the figure. The manufacturer's rating and a qualified installer settle the choice, particularly for combination systems.

Sources and review

Sizing a new water heater, 2017 archive, U.S. Department of Energy, 2017.
What size gas water heater do I need for my home?, Rheem, 2026.

Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.

How this is calculated

Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.

The arithmetic

Peak hour demand

Demand = Showers × 10 + Shaves × 2 + Hand washing × 4 + Dishwasher × 6 + Clothes washer × 7

Showers, and the rest: How many happen in your busiest hour
10, 2, 4, 6, 7: DOE's gallons of hot water per use

Worked through: 30 + 2 + 4 = 36 gallons

First hour rating

Rating to look for = Demand ± 2 gallons

2: DOE says to match within 1 or 2 gallons

Worked through: 36 − 2 = 34, and 36 + 2 = 38

The constants, and where they come from

SHOWER_GALLONS

Published range, narrowed

Hot water per shower, in gallons

Applies to: Standard use with no water conservation measures, as DOE's worksheet says