Free tool

Water Heater Size Calculator

Estimate the first-hour rating and tank size (or tankless flow) your home needs in seconds.

Instant estimate No sign-up One tap to a pro

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.

This is an estimate only. Call a local pro for an exact sizing and quote — every home is different.

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How water-heater sizing works

First-hour rating (FHR)

FHR is the gallons of hot water a unit can deliver in the busiest hour — the number that should match your peak demand, not just the tank size printed on the label.

Tank vs tankless

A tank stores a set number of gallons and refills between draws; a tankless unit heats on demand and is sized by flow (gallons per minute) for the fixtures running at once. We show both so you can compare capacity — never price.