Water heater sizing Last reviewed June 2026

Water Heater FHR Chart: First Hour Rating by Household Size

By the WhatSize editorial team 4 min read Standards-reviewed

TL;DR

The First Hour Rating (FHR) is the DOE-defined measure of how much hot water a water heater can deliver in an hour of typical use. For a 1-2 person household you need ~40-50 gal FHR. For 3-4 people: 50-65 gal. For 5+ people: 65-80 gal. Tankless is rated in GPM, not FHR — see the tankless chart below.

Looking for a different size? See What Size Water Heater for a 1-2 Person Household? or browse all water heater sizing guides.

How FHR is measured

DOE 10 CFR 430 defines FHR as the amount of hot water a tank water heater can supply in the first hour of use, starting with a fully-heated tank. The test simulates a typical draw pattern: a few minutes of shower, then a few minutes of dishwashing, then back to shower, etc. FHR accounts for both tank capacity and recovery rate (how fast the heating element or burner reheats water).

FHR is more useful than tank size alone because two water heaters with the same tank size can have very different FHRs if their recovery rates differ. A 40-gallon gas heater with a 40,000 BTU burner has a much higher FHR than a 40-gallon electric with a slow-recovery 4,500W element.

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FHR chart by household size

Tank water heater FHR lookup by household

Household size	Min FHR needed	Typical tank size
1-2 people	40-50 gal/hr	30-40 gal tank
3-4 people	50-65 gal/hr	40-50 gal tank
5-6 people	65-80 gal/hr	50-65 gal tank
7+ people or high use	80+ gal/hr	75-100 gal tank, or two tanks

How to find your existing water heater's FHR

The FHR is on the yellow ENERGY STAR label on the side of the water heater. It's also listed on the manufacturer's spec sheet and on the AHRI directory. If you can't find it, use the rule of thumb: a gas water heater's FHR is roughly equal to its tank size plus 10-20 gal; an electric water heater's FHR is closer to its tank size alone (slower recovery).

Tank vs tankless sizing

Tankless water heaters are sized by GPM (gallons per minute), not FHR. See our tankless GPM chart for the equivalent sizing. A whole-house tankless typically delivers 6-10 GPM, which can out-perform a 50-gallon tank during back-to-back showers.

Use our free water heater calculator for your household and usage pattern.

Frequently asked questions

What is a good FHR for a family of 4?

A family of 4 needs an FHR of at least 50-65 gallons per hour. A 40-50 gallon gas water heater typically achieves this. Electric water heaters in this size may need a 50+ gallon tank due to slower recovery.

Is FHR the same as tank size?

No. Tank size is the storage capacity (e.g., 50 gallons). FHR is the actual hot water delivered in the first hour, which depends on both tank size and recovery rate. A 50-gallon gas heater might have an FHR of 70+; a 50-gallon electric might have an FHR of 55.

How do I size a tankless water heater?

Tankless is sized in GPM, not FHR. Add up the GPM of all fixtures that might run simultaneously (typical: 1 shower 2.0 GPM + 1 faucet 1.0 GPM + dishwasher 1.5 GPM = 4.5 GPM). See our tankless GPM chart for details.