What Size Water Heater Do I Need?

The right water heater is sized to your home's busiest hour of hot-water use, not just the number of gallons on the label. Too small and you run out mid-shower; oversized and it heats water you never use. The goal is matching capacity to demand.

First-hour rating vs tank size

A tank's first-hour rating (FHR) is the gallons of hot water it can deliver in the busiest hour — stored gallons plus how fast it reheats. Two 50-gallon tanks can have different FHRs depending on the burner or element, so size to the FHR your household needs, not the tank label alone.

Estimate your size

Use the calculator at the top of this page to get a first-hour rating, a tank size to look for, and a tankless flow rate — then confirm with a local plumber.

Tank vs tankless

A tank keeps a set number of gallons hot and refills between draws. A tankless unit heats on demand and is sized by flow — gallons per minute (GPM) — for the fixtures running at once. A plumber can advise which fits your home and whether the existing gas, venting, or electrical will support it.

Next steps

Once you have a ballpark, a local plumber can confirm sizing, check your venting and connections, and handle the swap. One tap to call.