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Water Heater Size Calculator

Enter household size and hot-water usage to size your storage water heater tank.

How it is calculated

The tool works in US gallons. Peak-hour demand is people × rate, with a rate of 8 gal per person for low use, 12 for medium and 18 for high; any unrecognised usage level is treated as medium. The minimum tank is peak demand / 0.70, which allows for the part of the tank that is not usable hot water. The recommended size is the smallest standard tank of 30, 40, 50, 65, 75, 80, 100 or 120 gal that is at least the minimum. Above 120 gal the tool rounds the minimum up to the next 10 gal instead. At least one person is needed; fewer gives no result. Real heaters also differ by recovery rate and fuel, which the tool does not model.

Worked example

A household of 4 people with medium hot-water use. Peak-hour demand: 4 × 12 = 48 gal. Minimum tank: 48 / 0.70 = 68.57, shown as 69 gal. The first standard size that is at least 68.57 is 75 gal, so the recommended tank is 75 gal. The same four people with low use need 32 gal peak and 46 gal minimum, so a 50 gal tank. With high use the peak is 72 gal, the minimum 103 gal and the recommendation 120 gal.

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Common mistakes

  • Treating the figures as litres. They are US gallons, and one US gallon is about 3.785 litres, so 75 gal is roughly 284 litres.
  • Choosing the minimum tank instead of the recommended size. Tanks are sold in fixed sizes, so you buy the next one up.
  • Counting only people living in the home and ignoring a busy morning, when guests or teenagers take long showers. Choose a higher usage level for that case.
  • Expecting a precise answer for very large households. Above 120 gal the tool simply rounds up to the next 10 gal, and such loads usually need two heaters or a commercial unit.
  • Using the tank size alone and ignoring recovery rate. The tool sizes capacity only, not burner or element power.

About this calculator

Sizing a storage water heater comes down to matching its tank capacity and recovery rate to your household's peak-hour demand — the busiest single hour of hot-water use, like a morning stretch of back-to-back showers plus a dishwasher cycle. Undersizing leaves the last person in line with a cold shower; oversizing wastes standby energy keeping water hot that nobody uses. Manufacturers publish a first-hour rating (FHR) for exactly this reason, since a tank's usable output during a demand spike is higher than its nominal gallon capacity thanks to incoming cold water mixing and reheating as it draws down.

This calculator takes your household size and a low, medium or high hot-water usage level, estimates your peak-hour demand at roughly 8, 12 or 18 gallons per person, divides it by 0.70 to allow for the part of a tank that is actually usable, and recommends the next standard tank size up (30 to 120 gallons). Homeowners replacing an aging heater, plumbers quoting installations, and landlords sizing units for rental units use it to avoid buying a tank that runs out of hot water during the household's busiest moments — or paying for capacity that's never needed.

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Comments (1)

  • Editorial team

    Use the recommended tank as a starting point and then compare it with the first-hour rating printed on the heater's label, which should at least match your peak-hour demand. The tool does not model recovery rate or fuel type, so a smaller gas unit can sometimes keep up where an electric tank of the same size cannot. Figures are US gallons; 75 gal is about 284 litres. How many people share your hot water, and do showers usually overlap?

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