What Size Water Heater Do You Need? A Sizing Guide by Household
An undersized heater runs out of hot water. An oversized one wastes energy. Here is how to calculate the right size.
Safety Disclaimer
Water heater maintenance involves working with pressurized systems, scalding hot water, and potentially hazardous electrical or gas connections. Always shut off power (electric heaters) or gas supply (gas heaters) and allow water to cool to a safe temperature before beginning any maintenance. Wear appropriate safety equipment including gloves and eye protection. If you're uncomfortable with any step, contact a licensed plumber.
Tank Water Heater Sizing
The Quick Rule
| Household Size | Tank Size |
|---|---|
| 1-2 people | 30-40 gallons |
| 3-4 people | 40-50 gallons |
| 5+ people | 50-80 gallons |
The Precise Method: First Hour Rating (FHR)
The First Hour Rating is the number of gallons of hot water the heater can deliver in the first hour of use starting with a full, hot tank. This is a better metric than tank size because it accounts for recovery rate.
For more on this topic, see our guide on Electric vs. Gas Water Heaters: Cost, Efficiency, and Performance Compared.
To determine your needed FHR:
- Identify your peak hour of hot water use (usually morning).
- Add up gallons used during that hour.
For more on this topic, see our guide on Tank vs. Tankless Water Heaters: Which One Is Right for Your Home?.
| Activity | Gallons |
|---|---|
| Shower | 10-15 per person |
| Shaving | 2 |
| Hand/face washing | 2 |
| Dishwasher cycle | 6 |
| Clothes washer (hot) | 7 |
A family of four with two morning showers, one dishwasher load, and hand washing needs approximately 40-50 gallons FHR.
Tankless Water Heater Sizing
Flow Rate (GPM)
Tankless heaters are sized by gallons per minute (GPM) of hot water they can produce at a given temperature rise.
| Fixture | Flow Rate |
|---|---|
| Bathroom faucet | 0.5-1.5 GPM |
| Kitchen faucet | 1.0-1.5 GPM |
| Shower | 1.5-2.5 GPM |
| Dishwasher | 1.0-1.5 GPM |
| Clothes washer | 1.5-3.0 GPM |
Add up the GPM of fixtures you expect to run simultaneously. A typical household needs 5-7 GPM for gas tankless or 3-4 GPM for electric tankless.
Temperature Rise
Incoming cold water temperature affects sizing. In Minnesota (40°F incoming), you need more heating power than in Florida (70°F incoming). Most manufacturers provide sizing charts based on your climate zone.