What size water heater do I need?

One whose first-hour rating covers your busiest hour. Add up the showers, tub and appliances that run in that hour, and compare the total with the first-hour rating on the EnergyGuide label.

What is first-hour rating?

How many gallons of hot water a storage heater delivers in an hour starting full. It is on the yellow EnergyGuide label and decides which federal test pattern the heater is measured on.

Is a 50-gallon tank enough for four people?

Head count does not answer it. Four people showering at once and four showering hours apart need different heaters. Measure the busiest hour.

How do I find my busiest hour?

Pick the hour when the most hot water runs, such as a weekday morning. Count the showers, time one, read the showerhead rate, and add any tub or appliance filled in that hour.

Why is the answer an upper bound?

Showers count at full flow, but part of each shower is cold water mixed in. The hot water you use is less than the total shown.

What temperature should I set it to?

CPSC urges 120°F against scalds; CDC advises storing hot water above 140°F against Legionella. A thermostatic mixing valve lets you do both.

HyrePlumber tools

Water heater sizing calculator

Type the showers and appliances that run in your busiest hour. Nothing is stored or emailed.

2.0 gpm the most a WaterSense showerhead may use EPA WaterSense, Showerheads, retrieved September 5, 2026. The federal maximum is 2.5 gpm. Read the rate on your own showerhead.

Choose a storage water heater whose first-hour rating, on its EnergyGuide label, is at least the hot water your busiest hour uses. Two 8-minute showers at 2.0 gpm use 32 gallons, so look for a first-hour rating of 32 gallons or more. Tank size alone does not tell you.

Work out your busiest hour

Arithmetic on your figures. Nothing is stored or emailed.

— Look for

— Your busiest hour

— Federal test pattern

What this assumed —

Showers count at full flow, so this is an upper bound. A check on a bid, not a specification.

Worked examples

Two busy hours, run through the calculator

Two showers before work

Two 8-minute showers at 2.0 gpm = 32 gallons. Look for a first-hour rating of 32 gallons or more.

Three showers and a dishwasher

Three 10-minute showers at 2.5 gpm = 75 gallons, plus a dishwasher label of 4 gallons = 79 gallons. That is the High-Usage range of the federal test.

Peak hour = showers × minutes × showerhead gpm, plus tub, dishwasher and washer gallons in the same hour. The dishwasher figure is an example; read your label.

A 50-gallon tank is not 50 gallons of hot water an hour

Tank size is what the vessel holds. First-hour rating is how much hot water the heater delivers in an hour starting full, including what it reheats during that hour.

Recovery depends on the heat source. A burner or element adds hot water while you draw. Two 50-gallon heaters with different burners or elements can carry different first-hour ratings.

Watch a fuel change. Moving from gas to electric with the same tank size can mean less hot water in the busy hour. Compare first-hour ratings, not gallons.

Where to find it. The first-hour rating is printed on the yellow EnergyGuide label of a storage heater, next to the Uniform Energy Factor (UEF).

Where your first-hour rating lands in the federal test

The federal test places each storage heater on a draw pattern by its first-hour rating. Compare UEF numbers only between heaters on the same pattern.

Draw pattern	First-hour rating	Gallons per test day
Very-Small-Usage	0 to under 18 gal	10
Low-Usage	18 to under 51 gal	38
Medium-Usage	51 to under 75 gal	55
High-Usage	75 gal and above	84

US Government Publishing Office, Code of Federal Regulations, 10 CFR part 430, subpart B, appendix E (2022 edition), retrieved September 5, 2026.

How each type of heater changes the sizing

Gas storage

Recovers quickly for its size. Needs combustion air and a flue, which limits where it can go.

Electric storage

Recovers more slowly, so the first-hour rating sits closer to the tank size. Size on the rating.

Heat pump

Efficient, and slower to recover in heat-pump-only mode. Ask which mode the label’s first-hour rating reflects, and how much air space it needs.

Tankless

Not sized by first-hour rating. It is rated in gallons per minute at a temperature rise: use the tank vs tankless tool and ask the installer for a gpm at your winter rise.

The setting: scalds on one side, Legionella on the other

CPSC: “The U.S. Consumer Product Safety Commission (CPSC) urges all users to lower their water heaters to 120 degrees Fahrenheit.” At 140°F, CPSC says a third-degree burn can take six seconds.

CDC: “Legionella grows best between 77°F–113°F (25°C–45°C).” CDC advises storing hot water above 140°F.

Both, with a mixing valve. CDC names valves that mix hot and cold near the point of use. Store hot and deliver tempered water. Fitting one is a plumber’s job.

What to ask before you accept a size

What is the first-hour rating of the unit proposed?
From its EnergyGuide label, not the tank size.
What busy hour did you size it for?
Compare it with the figure from this calculator.
If the fuel is changing, what happens to recovery?
Ask for both first-hour ratings side by side.
Is a thermostatic mixing valve included?
And what temperature does it deliver?
Will it fit, with clearances?
Height, diameter, service space and, for a heat pump, air volume.
Is an expansion tank needed?
On a closed system, heated water has nowhere to expand. Ask whether one is in the bid.
Is the permit in the price?
Ask who pulls it and when it is inspected.
Never cap or plug the relief valve
The temperature and pressure relief valve is a safety device. If it discharges, call a plumber.

What the calculator leaves out

Mixing. Showers count at full flow, but part of every shower is cold water. The answer is an upper bound on hot water.

Incoming water temperature. Colder winter water slows recovery. Ask the installer about your winter conditions.

Fit and venting. Space, flue and electrical supply decide what can be installed.

Questions this calculator answers

What size water heater do I need?: One whose first-hour rating covers your busiest hour. Add up the showers, tub and appliances that run in that hour, and compare the total with the first-hour rating on the EnergyGuide label.
What is first-hour rating?: How many gallons of hot water a storage heater delivers in an hour starting full. It is on the yellow EnergyGuide label and decides which federal test pattern the heater is measured on.
Is a 50-gallon tank enough for four people?: Head count does not answer it. Four people showering at once and four showering hours apart need different heaters. Measure the busiest hour.
How do I find my busiest hour?: Pick the hour when the most hot water runs, such as a weekday morning. Count the showers, time one, read the showerhead rate, and add any tub or appliance filled in that hour.
Why is the answer an upper bound?: Showers count at full flow, but part of each shower is cold water mixed in. The hot water you use is less than the total shown.
Does this size a tankless heater?: No. Tankless is rated in gallons per minute at a temperature rise. Use the tank vs tankless tool and ask the installer for a gpm at your winter rise.
What temperature should I set it to?: CPSC urges 120°F against scalds; CDC advises storing hot water above 140°F against Legionella. A thermostatic mixing valve lets you do both.

Sources and methodology

Figures dated September 5, 2026. Last reviewed September 29, 2026.

Uniform Test Method for Measuring the Energy Consumption of Water Heaters (10 CFR 430, subpart B, appendix E, 2022) (US Government Publishing Office, Code of Federal Regulations, retrieved September 5, 2026). First-hour rating and the four draw patterns.
Showerheads (US Environmental Protection Agency, WaterSense, retrieved September 5, 2026). 2.5 gpm federal maximum, 2.0 gpm WaterSense maximum.
Tap Water Scalds (CPSC Publication 5098) (US Consumer Product Safety Commission, retrieved September 5, 2026). Burn times by temperature and the 120°F recommendation.
Monitoring Building Water (US Centers for Disease Control and Prevention, retrieved September 5, 2026). Storage above 140°F and mixing valves.
How to use the EnergyGuide label (Federal Trade Commission, retrieved August 23, 2026). Where first-hour rating is printed.

Water heater repair and replacement What a replacement includes and what the bid should say.
Tank vs tankless payback Whether tankless pays back on your bids.
Water heater repair vs replace Whether to replace at all.
Water usage calculator Your whole day, not the busiest hour.
Water hardness calculator Scale in the tank.

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