Water Heater Size Calculator
Water heater sizing is based on First Hour Rating (FHR) — how many gallons the unit can deliver in a busy morning hour. Enter your household size and peak usage to get the minimum FHR, recommended tank size, and estimated annual energy cost for gas and electric units.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
What size water heater do I need for a family of 4?
A family of 4 typically needs a First Hour Rating of 50–70 gallons. A 50-gallon gas or 55-gallon electric tank water heater covers most 4-person households. If you have two back-to-back showers in the morning, size up to a 60-gallon unit.
What is First Hour Rating (FHR)?
First Hour Rating is how many gallons of hot water a heater can deliver in a busy first hour, starting with a full tank. It accounts for heating capacity plus stored volume. The FHR appears on the EnergyGuide label. Match your peak demand to the FHR, not just the tank size.