What size water heater do I need?

The honest answer is not “50 gallons because you’re a family of four.” It is the DOE first-hour-rating method: size by your busiest hour of hot-water use, then match a tank whose First-Hour Rating (FHR) meets that peak — or a tankless sized for the peak flow.

Typical planning values. Your real hot-water use, incoming water temperature, recovery and efficiency vary by household, unit, fuel and region — confirm your unit’s rated First-Hour Rating, GPM and UEF on its EnergyGuide label and the manufacturer’s instructions. Round sizing up, and leave headroom for peak demand.

1 Enter your numbers

in the peak hour
About 20 gallons each (DOE typical).
in the peak hour
About 20 gallons each.
in the peak hour
in the peak hour
in the peak hour
in the peak hour
in the peak hour
in the peak hour
Your result
Recommended tank size50-gallon tank
Peak-hour hot-water demand75 gallons
Target First-Hour Rating≥ 75 gal
Tankless alternativesize for the peak GPM

The honest answer is the DOE first-hour-rating method — size by your busiest hour of hot-water use, not by headcount alone. This routine peaks at about 75 gallons, so a 50-gallon tank rated to a First-Hour Rating of 75 or more fits — or a tankless sized for the peak GPM at your temperature rise. Round up and leave headroom; confirm the unit’s rated FHR on its EnergyGuide label.

Every incumbent tool asks for headcount and spits back a round number. That gets people a tank that runs cold on a busy morning — or an oversized one that wastes standby heat all year. The variable that actually decides the size is the peak hour: the single 60-minute window when your household draws the most hot water at once. A family that staggers showers needs far less than one where three people leave within the same hour.

So this tool adds up your busiest hour from the DOE hot-water-use figures, then reads across to the tank whose First-Hour Rating covers it. FHR — not raw tank volume — is what the label promises you in that first hour, because a strong burner keeps reheating while you draw. Two 50-gallon tanks can post very different FHRs.

Formula

Peak-hour demand is the sum of every hot-water draw in your busiest hour:

peak_hour_demand = Σ(number_of_uses × gallons_per_use)

Then pick the tank whose First-Hour Rating ≥ peak_hour_demand (round up), or a tankless sized for the peak simultaneous flow. LABELED DOE draws: shower ≈ 20 gal, bath ≈ 20, shave ≈ 2, hand/face ≈ 4, shampoo ≈ 4, dishwasher ≈ 6, food prep ≈ 5, clothes washer ≈ 7.

Worked example

Take a family of four with a busy morning: 3 showers, 1 shave, 2 hand washes and one food-prep draw.

3×20 + 1×2 + 2×4 + 1×5 = 60 + 2 + 8 + 5 = 75 gallons

That 75-gallon peak points to a 50-gallon gas tank with a strong burner (FHR near 75), or a tankless sized for the peak GPM at your temperature rise. Notice the headcount (four) never entered the math — the routine did. If those same four people spread their showers across two hours, the peak halves and a 40-gallon tank is plenty.

Measure first, avoid a wrong size

Measure the real peak, not the average. The number that strands you in a cold shower is the worst hour, usually a weekday morning. Count only what overlaps in that window.

  • Round up, then add headroom. Guests, a growing family and a colder winter inlet all push demand up; a size of slack is cheap insurance against a cold tail-end.
  • Gas vs electric changes the answer. A gas burner recovers far faster than a 4,500-watt element, so an electric tank often needs more storage to hit the same FHR — check the recovery-time tool.
  • Tankless is sized differently. It has no FHR; it is rated by flow (GPM) at a temperature rise, so use the tankless sizing tool instead of a gallon figure.
  • Confirm the label. The tank’s rated First-Hour Rating is printed on its EnergyGuide sticker — buy to that number, not to the nominal gallons.

Reference table

LABELED DOE hot-water use per fixture — measure your own routine and adjust.

Fixture / useGallons per use
Shower20 gal
Bath (tub)20 gal
Shaving2 gal
Hand / face wash4 gal
Hair shampoo (sink)4 gal
Hand dishwashing4 gal
Automatic dishwasher6 gal
Food prep5 gal
Clothes washer7 gal

Frequently asked questions

What size water heater do I need for a family of 4?
For a typical family of four with a busy morning, the peak hour lands near 75 gallons (three showers plus incidental use), which a 50-gallon gas tank with a First-Hour Rating around 75 covers. A large electric tank may need 65–80 nominal gallons to match, because it recovers more slowly.
Is it the number of people or the number of bathrooms?
Neither on its own — it is how much hot water overlaps in your busiest hour. Bathrooms and people are proxies; the first-hour-rating method uses the actual draws, which is why staggering showers can drop you a whole tank size.
Should I just buy the biggest tank to be safe?
No. An oversized tank pays standby losses every hour of every day for water you never use, and costs more upfront. Size to your peak hour with a size of headroom, not to the top of the shelf.
How does this differ from a tankless recommendation?
A tankless has no stored gallons and no First-Hour Rating; it is sized by the flow (GPM) you draw at once at your temperature rise. Use the tankless sizing tool for that, then compare lifetime cost with the tank.

Related calculators

Peak-Hour Hot-Water DemandAdd up every hot-water draw in your busiest hour — showers, baths, shaves, dishwashers — to find the peak-hour demand that sets the First-Hour Rating you need.First-Hour-Rating CalculatorFirst-Hour Rating is the hot water a tank really delivers in hour one: usable storage plus recovery. Compute the FHR from your tank size and burner input.Size by HouseholdGet the recommended storage tank gallons, First-Hour Rating and tankless GPM band for your household size, then confirm it with the peak-hour demand method.How to Size a Water HeaterHow to size a water heater: add up the busiest hour of hot-water use, then pick a tank First-Hour Rating above it or a tankless GPM at your temperature rise.