Guide · 6 min read

What Size Water Heater Do I Need?

A plain-English guide to first-hour rating, tank size, and tankless flow.

Water Heater Size Calculator

First-hour rating, tank size & tankless flow
18
16
55 gal
First-hour rating (FHR)
~65 gal
Tank size to look for
~4 GPM
Tankless flow (alt.)

Estimates capacity for planning only — first-hour rating, tank gallons, and tankless flow. Confirm sizing with a local plumber. This tool does not provide pricing.

Open the full Water Heater Size Calculator

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.

Rule of thumb: match the FHR to your peak-hour demand within a few gallons — the calculator above estimates both from your household size.

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.

Talk it through with a local pro
We connect homeowners with independent local plumbers. Calls may be routed to a partner network; the number shown is a connection line, not a specific business.