A water heater is properly sized when it can meet the household’s busiest realistic period—not simply when its tank has a familiar gallon number. Choose storage or tankless mode below and enter the demand you expect at the same time.
Water-heater size calculator
Estimate the first-hour rating for a storage heater or the flow and temperature rise for a tankless model.
Use the model’s published first-hour rating or its flow-at-temperature-rise chart—not tank gallons or headline GPM alone. Installation, venting, gas and electrical work require a qualified professional.
Storage water-heater sizing
For a storage or heat-pump water heater, compare the calculator’s peak-hour demand with the model’s first-hour rating (FHR). The Department of Energy defines FHR as the amount of hot water a fully heated storage unit can supply in one hour. Tank capacity alone does not include recovery during that hour.
The usage estimates use the worksheet method in this DOE water-heating fact sheet. Efficient fixtures and appliance models may use less, while long showers and older equipment may use more. Use measured household behavior when you have it.
Tankless water-heater sizing
Tankless sizing depends on two linked values: the gallons per minute required by outlets that may run together and the temperature rise between incoming cold water and the desired outlet temperature. A colder winter supply creates a larger rise and reduces the flow a given unit can deliver.
DOE building guidance says tankless selection should use the practical maximum simultaneous flow and required temperature rise. Compare the result with the manufacturer’s flow-at-temperature-rise chart. The headline maximum GPM is usually measured at a smaller rise than a cold-climate home may need.
How the tankless estimate is calculated
The calculator adds the simultaneous fixture flows. Approximate heating input is then calculated as 500 × GPM × temperature rise for BTU/h; the electric equivalent divides that figure by 3,412. These are ideal heat-transfer estimates, so equipment efficiency, electrical service, gas supply and installation conditions still matter.
Frequently asked questions
Is a 50-gallon tank enough for four people?
It may be, but the answer depends on first-hour rating and clustered use. Compare the model’s FHR with the household’s busiest hour rather than choosing on nominal tank capacity alone.
Can this calculator specify an installation?
No. Water temperature, scald protection, venting, combustion air, gas piping, electrical capacity, drainage and local codes need a qualified professional and the manufacturer’s instructions.