Understanding your result
The headline is the tank to buy, with the peak-hour demand and the first-hour rating to look for in the sub-line. The stats repeat the demand, the rating and the tank. The table lists each use in the busiest hour with its gallons, the demand, and then every common tank of the type you chose with its estimated first-hour rating, ticked where it covers the demand.
The estimated first-hour ratings are for planning; the figure on the EnergyGuide label of a specific model is the one to match, and it can differ by several gallons from our estimate because burner sizes and tank designs vary. If no common tank covers the demand, the headline says so and the notes list the options.
The daily hot water use calculator gives the total for a day rather than the peak hour, which is what the running cost depends on; the recovery time calculator shows how fast a tank reheats between uses; and the tankless size calculator sizes the alternative. The replacement cost calculator prices the tank you choose.
How we calculate this
The per-use gallons are the US Department of Energy’s peak-hour worksheet figures from its “Sizing a new water heater” guidance; the bath figure of 20 gallons is our estimate, because tub sizes vary so much. The first-hour rating estimate treats about 60% of the tank as usable before the delivered water cools noticeably and adds one hour of recovery at the DOE test’s 67 °F rise, using typical inputs: 32,000 to 40,000 BTU per hour for residential gas tanks of 30 to 50 gallons, 75,000 for 75-gallon gas tanks, and a single 4,500 W element for electric and hybrid heat pump tanks. The energy of heating water and the conversions to litres are on the unit conversions page, and the UEF ratings that decide running cost on the efficiency ratings table.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Gallons per use | Shower 10, shave 2, hand wash 4, dishwasher 6, hand dishes 4, food prep 5, washer 7 | US DOE, Sizing a new water heater (peak-hour worksheet) |
| Bath | 20 gallons | Our estimate |
| Usable share of the tank | 60% | Our estimate, calibrated to typical label ratings |
| Test temperature rise | 67 °F | DOE first-hour rating test conditions |
| Gas tank inputs | 32,000 / 36,000 / 40,000 / 75,000 BTU/hr for 30 / 40 / 50 / 75 gal | Typical residential models |
| Electric and hybrid element | 4,500 W | Typical residential models |
| Recovery efficiency | 78 to 80% gas, 98% electric | Typical rating-plate values |
Assumptions last reviewed October 8, 2026.
The calculator does not model a water heater shared with a hydronic heating system, very long pipe runs that waste water waiting for it to get hot, low-flow fixtures that use less than the DOE figures, or hot water recirculation. The guide to water heater sizing covers first-hour ratings, tankless flow and temperature rise in detail, and the guide to water heater running costs covers what the size means for the bill.
Two worked examples
The DOE example household, gas
Three showers, one shave, four hand and face washes, one dishwasher load, one food preparation and one clothes washer load in the busiest hour; gas tank.
- Showers 30 + shave 2 + hand washing 16 + dishwasher 6 + food 5 + laundry 7 = 66 gallons
- 30 gal gas (32,000 BTU): first-hour rating about 63, too small
- 40 gal gas (36,000 BTU): about 74, covers it
- Tank: 40 gallons gas
A 40-gallon gas tank is right for this household; moving the laundry out of the morning rush would leave room for a fourth shower.
A couple with an electric tank
Two showers, one shave, two hand washes and one load of laundry in the busiest hour; electric tank.
- Showers 20 + shave 2 + hand washing 8 + laundry 7 = 37 gallons
- 30 gal electric (4,500 W): first-hour rating about 45, covers it
- Tank: 30 gallons electric; a 40-gallon tank (about 51) gives a margin
Even with an electric tank’s slow recovery, a small household is covered by a 30- or 40-gallon unit. If the household grows, the jump to 50 or 65 gallons matters more for electric than for gas.
Where to find your inputs
The busiest hour. For most households it is a weekday morning: count the showers, shaves and sink uses, and any dishwasher or laundry run then.
Type. The fuel you will use: gas tank, electric tank, or a heat pump water heater (sized here in hybrid mode, with its element helping).
First-hour rating of a model. On the yellow EnergyGuide label as “Capacity (first hour rating)”, and in the manufacturer’s specifications.
Common mistakes
- Sizing by tank gallons. The first-hour rating is what matters; a 50-gallon electric tank delivers less in an hour than a 40-gallon gas tank.
- Sizing by people alone. Two people who shower together in the morning need more than four who shower at different times.
- A large margin “to be safe”. Every extra gallon of tank loses heat all day.
- Forgetting the dishwasher and laundry. They add 13 gallons if they run in the busiest hour.
- Undersizing a heat pump water heater. It recovers slowly in efficiency mode; go one size larger.
- Ignoring groundwater. In winter colder inlet water cuts recovery; size for the coldest season.
Questions people ask
- What size water heater do I need?
- One whose first-hour rating, printed on the EnergyGuide label, at least matches your busiest hour of hot water use. Add up that hour using the DOE's figures of 10 gallons per shower, 6 per dishwasher load, 7 per clothes washer load, 2 per shave, 4 per hand and face wash and 5 for food preparation. The DOE's own example household comes to 66 gallons, which a 40-gallon gas tank with a 36,000 BTU burner covers.
- What is a first-hour rating?
- The number of gallons of hot water a tank water heater can supply in one hour starting with a full tank, measured under the Department of Energy test. It combines the usable stored water with what the burner or element can heat during the hour, so a gas tank with a strong burner has a higher first-hour rating than an electric tank of the same size. It is the right number for sizing; the tank's nominal gallons are not.
- Is a 40 or 50 gallon water heater better for a family of four?
- For gas, a 40-gallon tank usually covers a family of four whose busiest hour is three showers and a load of dishes; a 50-gallon tank adds a margin for a bath or a second load of laundry. For electric, which recovers much more slowly, the same family typically needs a 65- or 80-gallon tank, or a heat pump water heater of 65 gallons or more. The calculator shows the estimated first-hour rating of each size.
- Why do electric water heaters need bigger tanks?
- Because a 4,500-watt element delivers only about 15,000 BTU per hour, against 36,000 to 40,000 for a typical gas burner, so it reheats about 27 gallons an hour at a 67 °F rise against about 50 for gas. The first-hour rating of an electric tank is therefore mostly its stored water, and a larger tank is the way to get more. Heat pump water heaters in efficiency mode recover more slowly still.
- Can a water heater be too big?
- Yes, in running cost. A larger tank loses more heat through its walls all day whether you use the water or not, so an oversized tank costs a little more every month. It also costs more to buy and takes more space. Sizing to the first-hour rating rather than adding a large safety margin keeps standby losses down.
- Should I get a tankless water heater instead?
- A tankless unit is sized by the flow of the fixtures running at the same moment rather than by the hour, and it never runs out but can run short of flow when several fixtures are open in winter. It suits households with long but not simultaneous use and those short of space. The tankless size calculator sizes one for your fixtures and groundwater temperature.