What Size Water Heater Do I Need?
Enter your bedrooms and bathrooms to get the plumbing-code minimum for a tank water heater, or ENERGY STAR's minimum tank size for a heat pump model [1].
Water heater size calculator
This is the plumbing-code minimum first-hour rating — how much hot water the heater can deliver in one busy hour starting from a full tank. Look for it on the spec sheet (it's usually on the yellow EnergyGuide label too). Compare first-hour ratings, not just tank gallons: two tanks of the same size can deliver different amounts.
How water heaters are sized
Tank heaters are sized by first-hour rating, not tank size. The first-hour rating is how much hot water a heater delivers in one busy hour, starting with a full tank. It depends on both the tank size and how fast the heater reheats, so two tanks of the same size can deliver different amounts. Plumbing codes set a minimum based on bedrooms and bathrooms [1].
Heat pump heaters need bigger tanks. ENERGY STAR sets minimum tank volumes for them so the heat pump, not the backup electric elements, handles most of the demand [1].
Tankless heaters are sized by flow rate. They heat water as it flows, so what matters is how many fixtures run at once and how cold your incoming water is.
Go a size up for heavy use. ENERGY STAR recommends upsizing for households with teenagers or people who prefer baths [1]. For scale, the average American uses about 17 gallons of hot water a day [2].
Water heater sizing chart
Uniform Plumbing Code minimum first-hour rating and ENERGY STAR NextGen minimum heat pump tank size, in gallons [1].
| Bathrooms | Bedrooms | Min. first-hour rating | Heat pump min. tank |
|---|---|---|---|
| 1 to 1.5 | 1 | 38 | 36 |
| 1 to 1.5 | 2 | 49 | 45 |
| 1 to 1.5 | 3 | 49 | 59 |
| 2 to 2.5 | 2 | 49 | 45 |
| 2 to 2.5 | 3 | 62 | 59 |
| 2 to 2.5 | 4 | 62 | 72 |
| 2 to 2.5 | 5 | 74 | 72 |
| 3 to 3.5 | 3 | 62 | 59 |
| 3 to 3.5 | 4 | 74 | 72 |
| 3 to 3.5 | 5 | 74 | 72 |
| 3 to 3.5 | 6 | 74 | 72 |
Your local plumbing code governs. Check with your building department or plumber if you're unsure.
Sources
- ENERGY STAR — Heat Pump Water Heater Design Considerations (tank size guidelines) (accessed 2026-10-15)
- U.S. DOE Building America — Strategy Guideline: Proper Water Heater Selection (revised April 2015)
What will it cost?
See installed prices for tank, tankless, and heat pump water heaters in your state.
Water Heater Costs →Find a Water Heater Installation Pro Near You
Get free quotes from top-rated local pros. No obligation.
Sponsored — BuildCost may earn a referral fee at no extra cost to you. This site connects you with independent service professionals through a third-party matching platform.
Is your current one broken?
Check whether a repair or a replacement makes more sense.
Repair or Replace? →