Water Heater Size Calculator

Background.

Running out of hot water halfway through the second shower is a sizing problem, but rarely the one people think. The number on the side of the tank, 40 or 50 gallons, is not what decides it. What matters is how much hot water the heater can deliver in your busiest hour, and that depends on how fast it reheats as well as on how much it stores. The US Department of Energy's advice is to size a storage heater by its first-hour rating, the gallons it can supply in an hour starting from a full tank, and to compare that with the household's peak-hour demand.

Adding up that demand is straightforward. A morning with one shower at 20 gallons, 10 gallons through the kitchen faucet, a 6-gallon dishwasher cycle and a 15-gallon load of laundry comes to 51 gallons, so any tank with a first-hour rating of at least 51 gallons should keep up. A second 20-gallon shower in the same hour raises the requirement to 71, and the heater that coped before no longer does.

Tankless heaters store nothing, so they are sized the other way round: by how many gallons per minute must flow at once and how many degrees each gallon has to be heated. A 2-gallon-per-minute shower, a 1-gallon faucet and a 1-gallon dishwasher running together need 4 gallons a minute. Raising that from 50 °F groundwater to 120 °F is a 70-degree rise, about 140,000 BTU an hour of heat, which is why tankless models are rated by flow at a stated temperature rise and why the same unit delivers less in winter, when the water entering the house is colder.

What is water heater size calculator?

Water heater sizing matches what the heater can deliver to what the household draws at its busiest. For a storage tank, including heat pump models, the measure is the first-hour rating: the number of gallons of hot water the heater can supply in one hour starting with a full tank. It combines stored volume with recovery, the rate at which the burner, element or heat pump reheats incoming water, so a gas tank with a powerful burner can have a first-hour rating well above its tank volume, while a slow-recovering unit can fall below it. The rating appears on the EnergyGuide label and in the product specifications. Peak-hour demand is the matching figure for the house: the hot water used in the single busiest hour, not over the whole day. For a tankless or demand heater the measures are flow and temperature rise. Flow is the gallons per minute drawn by every hot tap and appliance that might run at the same moment; temperature rise is the difference between the hot water temperature you want and the temperature of the water coming in. Manufacturers publish the flow each model can sustain at various rises.

How to use this calculator.

1. 01For the busiest hour of a typical day, enter the hot gallons each activity uses and how many times it happens. A shower uses its flow rate times its length, so a 10-minute shower at 2 gallons a minute is 20 gallons. To leave an activity out, set its number of uses to 0; the gallons per use must stay above zero.
2. 02Enter the first-hour rating of a storage heater you are considering, from its EnergyGuide label or spec sheet. If you have no candidate in mind, set it to 0 to skip the storage check and read only the required rating.
3. 03For tankless sizing, enter the flow rates of the shower, faucet and dishwasher that could run at the same time. The showerhead's rating is usually stamped on it or in its manual. All three need a value above zero.
4. 04Enter the hot water temperature you want, usually 120 °F, and the temperature of the incoming cold water in the coldest month, which you can measure with a thermometer at a cold tap after letting it run.
5. 05Enter the flow a tankless candidate is rated for at your temperature rise, from the manufacturer's table. Leaving it at 0 skips the tankless check.

The formula.

Peak-hour demand is D = Σ (gᵢ × nᵢ): for each activity i, the hot gallons per use gᵢ times the number of uses nᵢ in the busiest hour, added up. The required first-hour rating is set equal to D, and the storage check passes when the candidate's first-hour rating is at least D. There is no safety margin built in, so a rating that only just covers the demand will run short on a morning when someone showers longer than usual.

The tankless side has two parts. Simultaneous flow F is the sum of the shower, faucet and dishwasher flow rates, in gallons per minute; the clothes washer is not included, and all three of the others always are. If you know your dishwasher never runs while someone showers, take its flow off the total when you read the result. The temperature rise is ΔT = desired temperature − incoming temperature, in °F. A tankless candidate passes when its flow at that rise is at least F. The calculator does not convert a heater's power rating into flow, which is why it asks for the flow the manufacturer quotes at your rise.

The physics behind those manufacturer tables is simple. A gallon of water weighs about 8.33 pounds and takes 1 BTU per pound to warm by 1 °F, so heating a flow of F gallons per minute by ΔT degrees takes about 500 × F × ΔT BTU per hour. At 4 gpm and a 70-degree rise that is 140,000 BTU an hour, about 41 kW. Halve the rise and the same heater can deliver roughly twice the flow, which is why the incoming water temperature matters as much as the showers and taps.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Department of Energy (2026). Sizing a New Water Heater. Energy Saver.
2. [2]U.S. Department of Energy (2026). Storage Water Heaters. Energy Saver.
3. [3]U.S. Department of Energy (2026). Tankless or Demand-Type Water Heaters. Energy Saver.
4. [4]Federal Trade Commission (2026). EnergyGuide Labeling: Appliance Labeling Rule. eCFR.
5. [5]National Institute of Standards and Technology (2026). NIST Handbook 133, Appendix E: General Tables of Units of Measurement. U.S. Department of Commerce.