Water heater size calculator

Start with the busiest 60 minutes in your home. Count showers, baths, sink use, and appliance draws during that hour. A tank that serves an ordinary day can still struggle when the same household concentrates its demand into a short period.

What the calculator returns

The main result estimates gallons drawn from the hot tank during your chosen hour. It rounds that total upward to create a first-hour-rating discussion target. This is an approximate heat-and-water balance, not a certified sizing method or a guarantee of uninterrupted hot water.

ENERGY STAR defines first-hour rating as the hot-water volume a fully heated storage heater can supply in an hour. Its standardized test conditions can differ from your inputs, so confirm the selected model's performance and local requirements. See the official product criteria and our first-hour-rating guide.

The peak-hour calculation

For showers, baths, and sink use, the calculator estimates the fraction coming from the hot tank:

Hot fraction = (mixed-use temperature − cold inlet temperature) ÷ (tank temperature − cold inlet temperature).

It multiplies each mixed-water volume by that fraction. Dishwasher and clothes-washer entries are already hot-water intake gallons, so they are added directly. Those appliances often heat water internally or use cold-only cycles; replace the example intake with the manufacturer's information or your observed cycle demand.

The model assumes stable temperatures and simple mixing. It does not simulate tank stratification, pipe losses, draw timing, recovery during an individual shower, recirculation, or mixing-valve performance. Temperature inputs describe assumptions; follow professional guidance for actual thermostat and anti-scald settings.

Household gallons are a separate heuristic

The nominal tank-volume starting bands are editorial screening assumptions: 30–40 gallons for one or two people, 40–50 for three or four, 50–65 for five or six, and 65–80 for larger households. These bands do not size a model. Two equal-volume tanks can have different recovery and delivery ratings.

Read what size water heater and water heater gallons before using household size as your only criterion. Large tubs, long showers, guests, or tightly grouped draws can outweigh head count.

Check heat-pump operation separately

For a heat-pump water heater, ask how the chosen tank and operating mode perform through the peak hour. Resistance backup can improve delivery while using more electricity. ENERGY STAR's design guidance discusses upsizing storage to reduce backup operation.

Take your demand worksheet to an installer who can compare model ratings, recovery, available space, and code. For instantaneous systems, switch to the tankless GPM calculator, which evaluates simultaneous flow instead of stored gallons.