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Water Heater Size Calculator: What Size Tank Do I Need?

Count the people at home and what runs in the busiest hour of the day, usually the morning showers. Choose how the tank is heated and the calculator finds the peak-hour demand and the smallest standard tank that covers it.

The heating power sets how fast the tank refills, shown on the rating plate.

Includes the waste margin explained below. Check your product label for coverage and sizes.

How the water heater size calculator works

A storage water heater rarely runs out on an average day; it runs out in the one hour when everybody showers back to back. That is why US labels quote a first-hour rating (FHR): the hot water a full tank can deliver in one hour, counting what the burner or element reheats meanwhile. The calculator adds up your busiest hour, then finds the tank whose first-hour output matches it.

Tank = max((peak-hour demand − recovery per hour) ÷ 0.7, 45 L × people), rounded up to a standard size

The 0.7 factor reflects that only about 70% of a tank comes out hot before cold inflow drops the temperature. Recovery is what the heat source can reheat in an hour for a 50 °C (90 °F) rise: about 52 L/h for a 3 kW element, 77 L/h (20 gal/h) for a 4.5 kW element, 157 L/h (41 gal/h) for a 40,000 BTU/h gas burner and roughly 30 L/h (8 gal/h) for a heat pump in heat-pump-only mode. The household floor of 45 L (12 US gal) per person stops a single-shower morning from producing an undersized tank for a large family.

Worked example

Four people with two showers and one bath in the morning rush, plus 2 gallons each at the basins, need 10 + 10 + 20 + 8 = 48 gal, or 181.7 L, in that hour. A 3 kW electric tank reheats 52 L of it, so the stored volume has to supply 129.7 L; divided by 0.7 that is a 185.3 L tank. The household floor is also 181.7 L, so the larger figure wins and the next standard size is 200 L, with an estimated first-hour output of 192 L. The same family in the US with three showers, a dishwasher and a warm wash (51 gal) needs a 50-gallon electric tank, rated around 55 gallons in the first hour.

Hot water per use

UseHot water from the tank
Shower (about 8 minutes)38 L / 10 gal
Bath76 L / 20 gal
Dishwasher cycle23 L / 6 gal
Warm or hot laundry load26 L / 7 gal
Hand washing, shaving, kitchen sink (per person)8 L / 2 gal

These are the figures used in US Department of Energy sizing worksheets, measured as water at tank temperature. A water-saving shower head at 6–8 L/min uses less; a rain shower at 12 L/min or more can double the figure.

Standard tank sizes

European electric cylinders come in 30, 50, 80, 100, 120, 150, 200 and 300 L, and their EU energy label states a load profile (S, M, L, XL) rather than a first-hour rating: L suits a household with a bath, XL a larger family. US tanks are sold in 30, 40, 50, 55, 65 and 80 gallons, with the FHR printed on the yellow EnergyGuide label. Compare that label with your peak-hour demand rather than choosing by tank size alone.

Sizing mistakes to avoid

  • Oversizing out of caution. A bigger tank loses more heat around the clock; one standard size above the result is plenty.
  • Treating a heat pump like an electric tank. Heat-pump tanks recover slowly in efficient mode, so they are usually chosen one size larger.
  • Sizing a tankless unit this way. Instantaneous heaters are sized by flow rate in L/min or gal/min and temperature rise, not by hourly volume.
  • Ignoring storage temperature. Many European guidelines store water at 60 °C to control Legionella, with a thermostatic mixing valve at the taps against scalding; US guidance often uses 49 °C (120 °F). Follow your local rules, and have gas and unvented tanks fitted by a qualified installer.

Frequently asked questions

What size water heater do I need for a family of 4?

Usually a 50-gallon electric or 40-gallon gas tank in the US, or 200 L in Europe. Check that the first-hour rating covers your busiest hour, around 45–55 gallons for four people showering in the morning.

What is a first-hour rating?

It is how many gallons of hot water a full tank delivers in one hour of use, including reheating. It is printed on the EnergyGuide label and is a better sizing figure than the tank volume.

Is a 40 gallon water heater enough for 3 people?

Often yes with gas, which recovers about 40 gallons per hour. With an electric element, a 40-gallon tank is tight if all three shower in the same hour; a 50-gallon tank is safer.

How many showers can a 50 gallon water heater provide?

About five or six back-to-back showers of 10 gallons each with an electric element, fewer with a rain shower head or if the dishwasher runs at the same time.

How big should a hot water cylinder be for 2 people?

In Europe, 80–120 L suits two people who shower; choose 150 L if you take baths. In the US, a 30- to 40-gallon tank is usual.

Last reviewed: 2026-09-27 · How we calculate