Quick answer: For a tank or heat pump water heater, add up the hot water your household uses in its busiest hour and choose a model with a first hour rating at least that high. For tankless, add the flow rates of fixtures used at once (a shower is about 2.5 gpm) and check the heater’s flow at your temperature rise, often 70°F.
Many people pick a water heater by tank size alone: 40 or 50 gallons. But two 50-gallon heaters can deliver very different amounts of hot water. The U.S. Department of Energy recommends sizing by peak demand instead.
Tank Water Heaters: Use the First Hour Rating
The first hour rating (FHR) is how many gallons of hot water a tank heater can supply in one hour starting full. It is printed on the yellow EnergyGuide label. A gas heater with a fast recovery may have a higher FHR than a larger electric tank.
Step 1: Find Your Peak-Hour Demand
Think about the busiest hour in your home, usually morning or evening, and count each use. The DOE uses these averages:
| Use | Gallons per use |
|---|---|
| Shower | 20 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Top-loading clothes washer | 25 |
| Front-loading (H-axis) washer | 15 |
Step 2: Add It Up
Three showers, one shave and one sink of dishes in an hour: 60 + 2 + 3 = 65 gallons. Add a top-loading load of laundry in the same hour and it becomes 90 gallons. Choose a heater whose first hour rating meets or beats your total.
| Busiest hour | Peak demand |
|---|---|
| 1–2 showers + sink use | about 25–45 gal |
| 3 showers + shave + sink | 65 gal |
| 4 showers + sink | 83 gal |
| 3 showers + top-load washer | 85 gal |
Skip the math: count your busiest-hour uses or your simultaneous fixtures and get the rating or flow rate you need. Open the free Water Heater Size Calculator →
Tankless Water Heaters: Flow Rate and Temperature Rise
- List the fixtures you might run at the same time. A showerhead uses about 2.5 gpm and a faucet about 0.75 gpm.
- Add the flow rates. One shower and one faucet = 3.25 gpm.
- Find the temperature rise: desired temperature minus incoming water temperature. With 50°F groundwater and 120°F hot water, the rise is 70°F.
- Choose a unit that delivers your total flow at your rise.
The DOE notes that typical units are rated around 5 gpm for gas and 2 gpm for electric models at a 70°F rise. In cold climates the incoming water is colder, so the rise is larger and the flow you get is lower.
Gas, Electric or Heat Pump?
- Gas tank: fast recovery and high first hour ratings.
- Electric tank: lower upfront cost, slower recovery.
- Heat pump: uses much less electricity; check its FHR in the efficient mode you will use.
- Tankless: endless hot water within its flow limit, but electric units need very large electrical service.
See what an electric water heater costs to run with the electricity cost calculator.
Typical Peak-Hour Demand by Household
Household size gives a rough idea, but your habits matter more. A family that showers in the evening and runs the dishwasher overnight has a lower peak than one where everyone showers between 6 and 7 a.m.
| Household | Typical busiest hour | First hour rating to look for |
|---|---|---|
| 1–2 people | 1–2 showers + sink | about 45–55 gal |
| 3 people | 2–3 showers + sink | about 55–65 gal |
| 4 people | 3–4 showers + sink | about 65–85 gal |
| 5+ people | 4+ showers + laundry | about 85 gal or more |
Use these as a starting point and adjust with the calculator for your own routine.
Recovery Rate and Fuel Type
The first hour rating depends on both tank size and how fast the heater reheats water (its recovery rate). Gas burners heat water faster than electric elements, so a 40-gallon gas tank often has a first hour rating similar to a 50-gallon electric tank. Heat pump water heaters are very efficient but recover slowly in efficiency mode; many switch to electric elements during heavy use.
Signs Your Current Water Heater Is Too Small
- Hot water runs out during back-to-back showers.
- You have to schedule laundry and dishwashing away from showers.
- The water turns lukewarm after 10–15 minutes.
- You added bathrooms, people or a large soaking tub since the heater was installed.
Tankless in Cold Climates
In northern states, winter groundwater can be 40°F or colder, which raises the temperature rise to 80°F or more. At that rise a tankless unit delivers less flow than its headline rating, so size for winter conditions or choose a model rated at your real rise.
Sources
Frequently Asked Questions
What size water heater for a family of 4?
Usually one with a first hour rating of about 65–85 gallons, depending on how many showers happen in your busiest hour.
Is a 40 or 50 gallon water heater better?
Compare first hour ratings rather than tank size. A 40-gallon gas heater can out-deliver a 50-gallon electric one.
How many showers can a 50 gallon water heater handle?
Roughly three to four back-to-back showers at 20 gallons each, depending on its first hour rating.
What size tankless water heater do I need?
Add the flow rates of fixtures used at once and size for your temperature rise; two showers at once need about 5 gpm.
How long does a water heater last?
Tank water heaters typically last about 8–12 years; tankless units can last longer with descaling.
Does a bigger water heater cost more to run?
Slightly, because a larger tank loses more standby heat, which is another reason to size by real demand.
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