Water heater size calculator (First Hour Rating)
Size a storage (tank) water heater the way the U.S. Department of Energy recommends — by First Hour Rating (FHR), the gallons of hot water it delivers in a busy first hour. Add up the hot-water uses in your single busiest hour and get the minimum FHR to look for on the EnergyGuide label.
65 gallons
First Hour Rating needed
- From showers
- 60 gallons
- From everything else
- 5 gallons
What this assumes
- Peak-hour demand = 3×20 + 1×2 + 1×3 + 0 = 65 gal.
- Look for a storage water heater with a First Hour Rating (FHR) at or above this number.
- Per-use gallons are DOE averages; low-flow fixtures use less.
Coverage rates & sources
Every number this calculator uses is a published engineering constant — not an estimate we made up. Here is exactly what it assumes and where each value comes from.
- Shower: 20 gal/useSource: U.S. DOE — Sizing a New Water Heater (energy.gov), peak-hour demand worksheet
- Shaving: 2 gal/useSource: U.S. DOE — Sizing a New Water Heater (energy.gov), peak-hour demand worksheet
- Hand dishwashing / sink: 3 gal/useSource: U.S. DOE — Sizing a New Water Heater (energy.gov), peak-hour demand worksheet
- Sizing method: First Hour Rating ≥ peak-hour demandSource: U.S. DOE — Sizing a New Water Heater (energy.gov)
Before you buy
- Sizes a STORAGE (tank) heater by First Hour Rating. Tankless (on-demand) heaters are sized by flow rate (GPM) and temperature rise — a different method; ask a plumber.
- Peak-hour demand is your single busiest hour, not your daily total. Match a heater whose FHR (on the EnergyGuide label) meets or beats it.
- Per-use gallons are DOE averages — a low-flow showerhead uses less; a soaking tub uses far more. Put baths, dishwashers, and laundry running in the same hour into 'other'.
- A licensed plumber confirms fuel type, venting, recovery rate, and code — especially for a gas-to-electric or tank-to-tankless change.
This is a planning estimate, not a substitute for a pro's on-site measurement. For load-bearing, structural, or code-regulated work, confirm quantities with a licensed contractor.
Frequently asked
How big a water heater do I need for a family of 4?
It depends on your peak hour, not your family size. If your busiest hour is 3 back-to-back showers, that's about 60 gallons of demand, so you'd want a First Hour Rating of 60+. This calculator follows the DOE's peak-hour worksheet so you size to your actual usage instead of a rule of thumb.