Water Heater Size Calculator
Size a tank or tankless water heater from household size and simultaneous showers. See gallons, first-hour rating, and tankless GPM.
How to use this water heater size calculator
Water heater size is first-hour rating (how many gallons of hot water in an hour) more than the tank stamp. A family of four often lands on 50 gallons gas or a slightly larger electric tank. Tankless is sized in gallons per minute at your winter incoming water temperature.
This is a planning tool, not a plumbing code calc. Cold groundwater needs more GPM or a bigger tank. Heat-pump water heaters use electric tank sizing with a different recovery — still start from occupancy. Match the EnergyGuide label to the first-hour number, not only gallons.
Formula
Worked example
Four people, gas tank, two showers at once: a 50-gallon tank with a first-hour rating near 70, or about 3.1 GPM tankless.
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Frequently asked questions
What size water heater for a family of 4?
Often 50 gallons gas or 50–65 electric, or a tankless unit around 3 GPM if two showers can run together.
What is first-hour rating?
How many gallons of hot water the unit can supply in an hour starting with a full tank. Buy to FHR, not just tank size.
Tank vs tankless?
Tanks are cheaper up front. Tankless saves space and standby loss but must cover peak GPM in winter.
Does climate matter?
Yes. 50°F inlet water needs more burner or element than 70°F inlet for the same shower temperature.
Is this a Manual J for water?
No. It is occupancy-based planning. A plumber should confirm flow, gas line, and venting.
Results are estimates for informational purposes and are not financial, legal, or professional advice. Last reviewed 2026-08-20.