Water Heater Size Calculator

1. Pick the water heater type. Choose storage tank to size by peak-hour demand, or tankless to size by flow rate and temperature rise.
2. For a tank, think of the busiest hour of your day, often the morning. Enter how many showers, baths, shaves and other uses happen in that hour. Edit the gallons per use if your fixtures differ.
3. For tankless, enter how many of each fixture could run at once and its flow rate. The flow is printed on many showerheads and aerators.
4. Set the target and incoming water temperature, and the gas efficiency from the spec sheet. Use the incoming temperature for your coldest month.
5. Optional: enter the Btu/h or kW rating of a unit you are considering. The calculator shows the most flow it can heat at your temperature rise.

Pick Metric to work in liters, L/min and C. The numbers you have typed are converted for you.

A storage tank is sized from the hot water your home uses in its busiest hour. Each use is counted and multiplied by the gallons it takes.

The Department of Energy advised choosing a First Hour Rating that matches within 1 or 2 gallons of your peak-hour demand. Picking a higher FHR gives you more margin.

Why 500? One Btu heats one pound of water by 1 F. A gallon of water weighs about 8.33 lb, and an hour has 60 minutes. So 8.33 x 60 = 499.8, which rounds to 500.

Gas units are rated by input, so divide by the efficiency. Electric resistance units turn almost all their power into heat, so kW = Btu/h / 3,412.

For metric work, the direct formula uses liters per minute and a rise in C:

The 0.0698 comes from 4.186 kJ per kg per C divided by 60 seconds. It gives results within about 0.2% of the 500 rule.

Storage Tanks Are Sized by Volume Over an Hour

A tank stores heated water and then keeps reheating. What matters is how much hot water it can deliver in a busy hour. That is the First Hour Rating on the yellow EnergyGuide label, shown as Capacity (first hour rating). Two 50 gallon tanks can have very different FHRs if one has a bigger burner.

Tankless Units Are Sized by Flow and Rise

A tankless unit heats water only as it flows. It must add the full temperature rise to every gallon, every minute. So it is sized by the gpm you run at once and how much it must warm the water. Cold incoming water and several showers at once push the size up fast.

Which Number to Compare

• Tank: compare your peak-hour demand with the FHR.
• Gas tankless: compare the required input with the unit input in Btu/h, or use the reverse check.
• Electric tankless: compare the required kW with the unit kW, and check your electric service can supply it.

These are the per-use values from the classic Department of Energy peak-hour worksheet. They assume no water-saving fixtures.

The current Department of Energy page uses larger figures for some uses, such as 20 gallons per shower. Try the Longer showers chip to see that case.

The next table shows the heat a tankless unit must add. Gas input assumes 0.95 efficiency.

The incoming water temperature sets the rise, and the rise drives tankless size. The Department of Energy suggests assuming 50 F (10 C) unless you know otherwise.

Groundwater temperature varies by region and by season. Water in a northern winter can be much colder than water in a southern summer. To measure yours, run a cold tap for a minute and hold a thermometer in the stream.

Size a tankless unit for your coldest month. At 43 F, a 120 F target needs a 77 F rise. The same heater then delivers about 9% less flow than at a 70 F rise.

Storage Tank

1. In the busiest hour, three people shower: 3 x 12 = 36 gallons.
2. One shave is 2 gallons, one shampoo is 4 gallons and one hand dishwashing is 4 gallons.
3. Peak-hour demand = 36 + 2 + 4 + 4 = 46 gallons (174.1 L).
4. Look for a tank whose FHR is within 1-2 gallons of 46 or higher, so 44 gallons or more.

Gas Tankless

1. A 2.5 gpm shower and a 1.5 gpm faucet run together: 4.0 gpm.
2. Incoming water is 50 F and the target is 120 F, so the rise is 70 F.
3. Output = 4.0 x 500 x 70 = 140,000 Btu/h.
4. At 0.95 efficiency, gas input = 140,000 / 0.95 = 147,368 Btu/h. An electric unit would need 140,000 / 3,412 = 41.0 kW.

Checking a 199,000 Btu/h Unit

1. Two 2.0 gpm WaterSense showers and a 1.5 gpm faucet make 5.5 gpm. At a 70 F rise, that needs 192,500 Btu/h of output and 202,632 Btu/h of input.
2. A 199,000 Btu/h unit at 0.95 delivers 199,000 x 0.95 / (500 x 70) = 5.40 gpm. That is just short of 5.5 gpm, so the water would run slightly cooler.

Size a tank by its peak hour, not by the number of people. If four people shower in the same hour (4 x 12 = 48 gallons), one person shaves (2 gallons) and someone washes dishes by hand (4 gallons), the peak-hour demand is 54 gallons. Choose a tank whose First Hour Rating is within 1-2 gallons of 54 or higher, so about 52 gallons or more.

The First Hour Rating (FHR) is how many gallons of hot water a storage water heater can supply in one hour, starting with a full tank. It depends on the tank size and the burner or element. It is printed on the yellow EnergyGuide label as Capacity (first hour rating). Compare the FHR, not the tank size, with your peak-hour demand.

Add the flow rates of the fixtures you expect to run at the same time, then find the temperature rise: target temperature minus incoming water temperature. Required output in Btu/h = gpm x 500 x rise. For a 2.5 gpm shower and a 1.5 gpm faucet (4.0 gpm) with 50 F water heated to 120 F, the output is 140,000 Btu/h, or 147,368 Btu/h of gas input at 0.95 efficiency.

One Btu raises one pound of water by 1 F. A gallon of water weighs about 8.33 pounds, and there are 60 minutes in an hour, so 1 gpm is about 8.33 x 60 = 499.8 pounds per hour. Rounding that to 500 gives Btu/h = gpm x 500 x temperature rise.

The U.S. Department of Energy suggests assuming 50 F (10 C) unless you know otherwise. You can measure it by holding a thermometer under a cold tap after the water has run. Groundwater temperature changes with region and season, so size a tankless heater for your coldest month. At 43 F, a 120 F target needs a 77 F rise instead of 70 F.

Divide the required output in Btu/h by 3,412, the number of Btu/h in one kilowatt. A 4.0 gpm flow with a 70 F rise needs 140,000 Btu/h, which is 41.0 kW. That is why the Department of Energy notes that electric units typically manage only about 2 gpm at a 70 F rise.

Multiply the input by the efficiency, then divide by 500 times the rise. At 0.95 efficiency and a 70 F rise, 199,000 x 0.95 / (500 x 70) = 5.40 gpm. With colder 43 F water (a 77 F rise), the same unit delivers about 4.91 gpm.

• U.S. Department of Energy, Energy Savers: Sizing Storage and Heat Pump (with Tank) Water Heaters (archived December 2010) (peak-hour worksheet values per use; choose an FHR that matches within 1 or 2 gallons of peak-hour demand).
• U.S. Department of Energy, Energy Saver: Sizing a New Water Heater (archived March 2025) (tankless sizing by summed flow and temperature rise; assume 50 F incoming water; 120 F for most uses).
• University of Florida IFAS Extension: Energy Efficient Homes: Water Heaters (FCS3277) (FHR within 1 to 2 gallons of peak-hour demand; FHR on the EnergyGuide label).
• U.S. EPA WaterSense: Showerheads and Bathroom Faucets (labeled showerheads 2.0 gpm or less; labeled bathroom faucets 1.5 gpm or less).
• 10 CFR 430.32, Energy and water conservation standards (showerheads 2.5 gpm at 80 psi; lavatory and kitchen faucets 2.2 gpm at 60 psi).
• University of Nebraska-Lincoln Extension: Private Drinking Water Wells: Planning for Water Use (G2149) (typical flow for operation: dishwasher 2 gpm, automatic washer 5 gpm).