Water Heater Size Chart: Tank Gallons by Household

How the tank size is worked out

Three steps. First, list everything that uses hot water in the busiest hour of the day — usually the morning, when showers, the dishwasher, and the washer all run at once. Second, add up the gallons: a shower is 10 gallons, a bath 15, a dishwasher load 6, a clothes washer load 7, hand dishwashing 4, food prep 5, shaving 2, and hair washing 4. Third, match the total to a tank: the tank’s first-hour rating, the gallons of hot water it can deliver in one hour, should be at least the peak-hour demand. The bands are 36 gallons or less for a 30-gallon tank, 37 to 47 for a 40-gallon tank, 48 to 57 for a 50-gallon tank, 58 to 67 for a 65-gallon tank, and 68 or more for a 75–80 gallon tank. At the defaults the peak hour is 2 showers at 10 gallons each, a dishwasher load at 6, a washer load at 7, food prep at 5, and shaving at 2 — 20 plus 6 plus 7 plus 5 plus 2 is 40 gallons — so the first-hour rating target is 40 and the tank is 40 gallons.

The worked example, line by line

Setup: a 4-person household, peak hour in the morning.

Line 1 — showers. 2 showers times 10 gallons is 20 gallons.

Line 2 — dishwasher. 1 load times 6 gallons is 6 gallons.

Line 3 — clothes washer. 1 load times 7 gallons is 7 gallons.

Line 4 — food prep. 1 session times 5 gallons is 5 gallons.

Line 5 — shaving. 1 shave times 2 gallons is 2 gallons.

Line 6 — peak-hour demand. 20 plus 6 plus 7 plus 5 plus 2 is 40 gallons.

Line 7 — first-hour rating target. The target equals the demand: 40 gallons.

Line 8 — tank size. 40 falls in the 37–47 band, so the tank is 40 gallons.

Hot water per use

Every recommendation on this page is built from this table. These are the standard per-use gallons used in US Department of Energy–style sizing guidance, in our own words.

The size chart, by household

Each row shows a typical peak hour for that household size, built from the per-use table above, with the arithmetic shown. Your own peak hour may differ — use the calculator above for your exact uses.

Every branch, checked

1 person: 1 shower (10) plus food prep (5) plus shaving (2) is 17 gallons in the peak hour. 17 is in the 36-or-less band, so the tank is 30 gallons — the smallest standard tank, and plenty for one person.

6 people: 4 showers (40) plus a bath (15) plus a dishwasher load (6) plus a washer load (7) plus food prep (5) plus shaving (2) is 75 gallons in the peak hour. 75 is in the 68-or-more band, so the tank is 75–80 gallons — the largest standard residential size.

Add a bath to the default: the default peak hour is 40 gallons; a bath adds 15, so the demand becomes 55 gallons. 55 is in the 48–57 band, so the tank is 50 gallons — one step up from the 40-gallon default.

3 people: 2 showers (20) plus a dishwasher load (6) plus food prep (5) plus shaving (2) is 33 gallons. 33 is in the 36-or-less band, so the tank is still 30 gallons — three people can share a 30-gallon tank if their peak hour stays this light.

5 people: 3 showers (30) plus a dishwasher load (6) plus a washer load (7) plus food prep (5) plus shaving (2) is 50 gallons. 50 is in the 48–57 band, so the tank is 50 gallons.

Gas vs electric: electric tanks recover slower than gas — they heat the replacement water more slowly after the tank is drawn down — so the guidance is to size up one step for electric when the household sits at the top of a tank’s range. A 47-gallon peak hour fits a 40-gallon gas tank but is at the very top of its band, so an electric household at 47 gallons should take the 50-gallon tank instead. This is guidance, not a calculation: the calculator does not adjust for fuel type.

What the defaults assume

The defaults are a typical 4-person morning: 2 showers, a dishwasher load, a washer load, food prep, and shaving, all inside one hour, for 40 gallons. The per-use gallons are the standard figures from US Department of Energy–style sizing guidance: shower 10, bath 15, dishwasher 6, clothes washer 7, hand dishwashing 4, food prep 5, shaving 2, hair washing 4. The tank bands are first-hour rating bands, not tank volumes alone: a tank’s first-hour rating depends on its size and its recovery rate, so two 40-gallon tanks can have different ratings. The chart sizes by demand, not by brand, and it assumes the peak hour is the binding constraint — if your household’s hot water use is spread evenly through the day, a smaller tank can work, and if everyone showers back-to-back, the peak hour is the number that matters.

Rounding convention, stated plainly: all gallons are whole numbers — the per-use figures are whole gallons, so the peak-hour demand is a whole number, and the tank sizes are standard whole-gallon sizes. No rounding is hidden anywhere: 20 plus 6 plus 7 plus 5 plus 2 is exactly 40.

Output · formula · meaning

Who uses this

• The new homeowner: moves into a 4-person household and learns the peak hour is 40 gallons — 2 showers, a dishwasher load, a washer load, food prep, and shaving — so a 40-gallon tank is the right size, not the 50-gallon one the installer suggested.
• The solo renter: lives alone and learns 1 shower, food prep, and shaving add up to only 17 gallons in the peak hour, so a 30-gallon tank is plenty — and a bigger tank would just waste energy keeping water hot.
• The big family: has 6 people and learns 4 showers, a bath, a dishwasher load, a washer load, food prep, and shaving add up to 75 gallons in the peak hour, so the 75–80 gallon tank is the only size that covers the morning rush.
• The remodeler: adds a bathroom with a tub and learns the bath alone adds 15 gallons to the peak hour — 40 becomes 55 — so the 40-gallon tank has to become a 50-gallon one before the new bathroom goes in.

Accuracy & limitations

The math is exact for the numbers you enter: the peak-hour demand is the per-use gallons added up, the first-hour rating target is the demand itself, and the tank is read off the bands — 36 or less for 30 gallons, 37 to 47 for 40, 48 to 57 for 50, 58 to 67 for 65, 68 or more for 75–80. But the inputs carry the assumptions. The per-use gallons are standard planning figures, not measurements of your fixtures: a low-flow showerhead uses less than 10 gallons, a long shower uses more, and a front-load washer uses less hot water than a top-load one. The first-hour rating varies by model — two 40-gallon tanks can have different ratings because their recovery rates differ — so check the yellow EnergyGuide label on the tank you are actually buying and compare its first-hour rating with your demand, not just its gallon size. This chart sizes by demand, not by brand, and it assumes the peak hour is the binding constraint. It does not account for simultaneous uses beyond the peak hour, for a household that runs the dishwasher and washer at night instead of in the morning, or for a tankless heater, which is sized by flow rate instead of gallons. For a tankless comparison, see the tank vs tankless water heater page.