Water Heater Size Calculator

About this calculator

Sizing a storage water heater comes down to matching its tank capacity and recovery rate to your household's peak-hour demand — the busiest single hour of hot-water use, like a morning stretch of back-to-back showers plus a dishwasher cycle. Undersizing leaves the last person in line with a cold shower; oversizing wastes standby energy keeping water hot that nobody uses. Manufacturers publish a first-hour rating (FHR) for exactly this reason, since a tank's usable output during a demand spike is higher than its nominal gallon capacity thanks to incoming cold water mixing and reheating as it draws down.

This calculator adds up typical hot-water draws for showers, baths, dishwashing and laundry based on your household size and habits, then recommends a tank size in gallons that covers your peak hour with margin. Homeowners replacing an aging heater, plumbers quoting installations, and landlords sizing units for rental units use it to avoid buying a tank that runs out of hot water during the household's busiest moments — or paying for capacity that's never needed.