Sizing Your Water Heater
Updated February 24, 2026
An undersized water heater runs out during showers. An oversized one wastes energy. Tank heaters are sized by first-hour rating (FHR). Tankless by flow rate (GPM) and temperature rise. Getting it right means never running cold.
Overview
An undersized water heater runs out during showers. An oversized one wastes energy. Tank heaters are sized by first-hour rating (FHR). Tankless by flow rate (GPM) and temperature rise. Getting it right means never running cold.
What to Know
Tank Water Heaters
- First-hour rating (FHR) is the key metric -- how much hot water delivered in the first hour of heavy use.
- Calculate peak-hour demand: showers (20 gal each), dishwasher (6 gal), clothes washer (7 gal), faucets (4 gal each).
- Match FHR to that number.
Tankless Water Heaters
- Sized by flow rate (GPM) and temperature rise.
- Add GPM of all simultaneous fixtures.
- Calculate rise: desired output (110-120F) minus incoming water temp (40-70F by region).
- Match unit to both numbers.
Quick Sizing Guide
- 1-2 people: 30-40 gal tank or small tankless (6-7 GPM gas).
- 2-3 people: 40-50 gal.
- 3-4 people: 50-65 gal or mid tankless (8-9 GPM gas).
- 5+: 65-80 gal or large tankless (10-11 GPM gas).
- Heat pump: size up one tier.
Comparison
| 30-40 gal: 1-2 people, small households |
| 40-50 gal: 2-3 people, moderate use |
| 50-65 gal: 3-4 people, most families |
| 65-80 gal: 5+ people or high demand |
| Tankless: sized by GPM and temperature rise, not gallons |
Buying Tips
- Use first-hour rating (FHR), not just tank size, to compare.
- Cold climates need tankless sized for higher temperature rise.
- Heat pump water heaters recover slower -- size up.
- When in doubt, size up. Slightly oversized costs marginally more but never runs cold.
Common Mistakes
- Sizing by tank gallons alone -- FHR matters more.
- Buying tankless sized for warm climate in a cold climate.
- Replacing 40-gal with 40-gal without checking if the old one was adequate.
- Forgetting simultaneous use -- two showers need 4-5 GPM total.
Bottom Line
Tank: match FHR to peak-hour demand. Tankless: match GPM and temperature rise. Size up when in doubt.
Frequently Asked Questions
What should I look for when buying sizing your water heater?
Use first-hour rating (FHR), not just tank size, to compare. Cold climates need tankless sized for higher temperature rise. Heat pump water heaters recover slower -- size up.
What are common mistakes when buying sizing your water heater?
Sizing by tank gallons alone -- FHR matters more. Buying tankless sized for warm climate in a cold climate. Replacing 40-gal with 40-gal without checking if the old one was adequate.
What is the bottom line on sizing your water heater?
Tank: match FHR to peak-hour demand. Tankless: match GPM and temperature rise. Size up when in doubt.
What's the bottom line on sizing your water heater?
Tank: match FHR to peak-hour demand. Tankless: match GPM and temperature rise. Size up when in doubt.
What are the top tips for sizing your water heater?
Use first-hour rating (FHR), not just tank size, to compare.. Cold climates need tankless sized for higher temperature rise.. Heat pump water heaters recover slower -- size up..
What mistakes should I avoid when buying?
Common mistakes: Sizing by tank gallons alone -- FHR matters more.. Buying tankless sized for warm climate in a cold climate.. Replacing 40-gal with 40-gal without checking if the old one was adequate..
What are the best water heaters brands?
Top brands include AO Smith, Bosch, Bradford White, Ecosmart, Eemax, Eternal. Each serves different price points and needs.