Energy
Water Heater Size Calculator
Size a tank or tankless water heater to your household's peak hot water demand and fixture count.
Your answer
Based on simultaneous fixture use during your busiest single hour, typically weekday mornings.
Sizing by peak-hour demand, not household size alone
The most common water heater sizing mistake is matching tank size to number of bedrooms or people, without considering when hot water is actually used. A household of four that showers one at a time, spread across an hour, needs far less capacity than the same household with everyone showering back-to-back before work and school.
The correct approach uses first-hour rating (FHR) — the amount of hot water a tank heater can deliver in a single busy hour, accounting for both stored hot water and how fast the heating element or burner can recover. Add up realistic simultaneous or back-to-back fixture use during your household's single busiest hour, and match a tank whose FHR meets or slightly exceeds that number.
Typical hot water use per fixture
| Fixture | Typical hot water use | Notes |
|---|---|---|
| Shower | 10-15 gal | Longer or rain-head showers use more |
| Bath | 18-25 gal | Larger soaking tubs use more |
| Dishwasher | 4-6 gal per load | Efficient models use less |
| Washing machine (hot cycle) | 7-10 gal per load | Most loads run warm or cold today |
Tank vs. tankless sizing logic
Tank water heaters are rated by first-hour rating, which reflects a fixed volume plus recovery. Tankless units work differently — they heat water on demand and are rated by maximum flow rate in gallons per minute (GPM) they can raise to target temperature simultaneously. A tankless unit sized for one shower will bottleneck if a dishwasher starts at the same time, so tankless sizing needs to account for true simultaneous use, not just back-to-back use.
| Household size | Typical tank size | Typical tankless flow needed |
|---|---|---|
| 1-2 people | 30-40 gal | 5-6 GPM |
| 2-3 people | 40 gal | 6-7 GPM |
| 3-4 people | 50 gal | 7-8 GPM |
| 5+ people | 50-80 gal or tankless | 8-10 GPM |
Steps to size your own water heater
- List every hot water fixture used during your household's single busiest hour, typically weekday mornings.
- Add up realistic gallons used per fixture during that hour, including back-to-back showers.
- Match that peak-hour total to a tank's first-hour rating, or to a tankless unit's simultaneous flow rate.
- Add a modest buffer (about 10%) for guests, seasonal changes, or future household growth.
- Check your fuel type's practical recovery speed — gas recovers faster than standard electric, which affects how much stored volume you actually need.
How do I size a water heater for my household?
Use first-hour rating for tank heaters: add up the hot water your household uses in its busiest single hour (usually morning showers), and choose a tank whose first-hour rating meets or slightly exceeds that number.
Is a tankless water heater always better for a big household?
Not necessarily. Tankless units are rated by flow rate (gallons per minute) they can heat simultaneously — a large household running two showers and a dishwasher at once needs a high-flow-rate unit, which costs more and may need bigger gas or electrical service.
What size tank do most homes actually need?
A 40-gallon tank commonly serves 2-3 people, a 50-gallon tank serves 3-4 people, and households of 5 or more typically need 50-80 gallons or a tankless system sized to peak simultaneous demand.
Does a bigger tank always mean higher energy bills?
A larger tank has more standby heat loss, but modern insulated tanks lose relatively little. Right-sizing to avoid running out of hot water usually matters more for satisfaction than the small standby-loss difference between adjacent tank sizes.
Last reviewed February 2026