What Size Water Heater Do I Need?
For tank water heaters: 1-2 people need 30-40 gallons, 3-4 people need 40-50 gallons, and 5+ people need 50-80 gallons. For tankless: measure by GPM (gallons per minute) — a shower uses 2 GPM, a dishwasher 1.5 GPM. Most homes need a tankless unit rated for 7-10 GPM to handle simultaneous demands.
Cost Breakdown
| Service | Low | High | Notes |
|---|---|---|---|
| 30-40 gallon tank | $400 | $800 | 1-2 people |
| 40-50 gallon tank | $500 | $1,000 | 3-4 people, most common |
| 50-65 gallon tank | $600 | $1,200 | 5-6 people |
| Tankless (5-7 GPM) | $1,000 | $2,000 | Small-medium home |
| Tankless (8-11 GPM) | $1,500 | $3,000 | Large home, cold climate |
Water Heater Sizing Guide
Tank Water Heater Sizing
Tank water heaters are sized by storage capacity (gallons). The right size depends on how many people use hot water and peak demand (when multiple fixtures run simultaneously).
- 1-2 people: 30-40 gallon tank
- 3-4 people: 40-50 gallon tank (most common)
- 5-6 people: 50-65 gallon tank
- 7+ people: 65-80 gallon tank or consider two units
The First Hour Rating (FHR) on the EnergyGuide label is more important than tank size. It measures how many gallons of hot water the unit can deliver in the first hour of use. Match it to your peak demand.
Tankless Water Heater Sizing
Tankless units are sized by flow rate (GPM) and temperature rise. Add up the GPM of all fixtures you might use simultaneously:
- Shower: 2.0-2.5 GPM
- Kitchen faucet: 1.5 GPM
- Bathroom faucet: 1.0 GPM
- Dishwasher: 1.5 GPM
- Washing machine: 2.0 GPM
If you might run a shower (2.5 GPM) + dishwasher (1.5 GPM) + kitchen faucet (1.5 GPM) simultaneously, you need a unit rated for at least 5.5 GPM.
Temperature Rise
Incoming water temperature matters. In warm climates (incoming 60-70°F), a smaller unit works. In cold climates (incoming 40-50°F), you need a larger unit to achieve the same output. Check your incoming water temperature with a thermometer.
Related Questions
What happens if my water heater is too small?
You will run out of hot water during peak usage. If your tank recovers within 30 minutes and you can plan around it, you may be fine. If you consistently run out, upgrade to a larger tank or switch to tankless.
Can a water heater be too big?
Yes. An oversized tank wastes energy heating water you do not use. Standby heat loss is proportional to tank size. An oversized tankless unit is fine — it simply modulates down and does not waste energy.
How do I find my water heater's capacity?
Check the label on the side of the tank. It lists the capacity in gallons, the First Hour Rating, and the EnergyGuide information. If the label is missing, measure the tank height and diameter to estimate.