About the Water Heater Size Calculator
This water heater size calculator estimates your household’s peak-hour hot water demand — the most hot water you use in any single hour, usually the morning rush — and recommends a tank size whose first hour rating (FHR) can meet it. Enter how many showers, shaves, sink sessions, dishwasher and washing machine loads happen in that busiest hour, and pick gas, electric or heat pump.
The FHR, printed on the EnergyGuide label (or the spec sheet for heat pump models, which the FTC does not require to carry the label), is how many gallons of hot water a full tank can deliver in an hour of use. It depends on tank size and how fast the burner or element reheats, which is why a 40-gallon gas heater often out-delivers a 50-gallon electric model. Matching FHR to peak demand, rather than tank volume to family size, is the sizing method the U.S. Department of Energy recommends.
Per-use gallons follow the DOE Energy Saver worksheet: 20 gal of hot water per shower, 2 per shave, 3 for hand dishwashing or food prep, 7 per dishwasher load and 25 (top-loader) or 15 (front-loader) per clothes-washer load. Lower the shower figure if you have low-flow heads and short showers. FHR values by size are typical of current models; always check the label of the specific heater, and consider a tankless unit if you need endless hot water at a steady rate.
How to use the water heater size calculator
- 1Pick the type of water heater you are considering.
- 2Think of your busiest hour of hot water use, usually weekday mornings.
- 3Enter how many showers, shaves, sink sessions and appliance loads happen in that hour.
- 4Read the peak-hour demand and the minimum first hour rating.
- 5Compare FHR values on EnergyGuide labels or spec sheets and choose a model at or above it.
Formula and method
Peak-hour demand adds up the hot water used in your household’s busiest hour, using the average hot-water volumes in the U.S. Department of Energy’s Energy Saver worksheet: 20 gal per shower, 2 per shave, 3 for hand dishwashing or food preparation, 7 per automatic dishwasher load, and 25 gal for a top-loading or 15 gal for a front-loading (horizontal-axis) clothes washer. You can lower the shower figure to match low-flow fixtures.
A storage water heater can supply that hour if its first hour rating (FHR) — the gallons of hot water it can deliver in one hour starting from a full tank, as tested under the DOE procedure — is at least the peak demand. Gas burners reheat water faster than electric elements, so gas tanks reach a given FHR with a smaller tank. The calculator picks the smallest common size of your chosen type whose typical FHR meets the demand.
- FHR
- First hour rating in gallons (EnergyGuide label)
- Peak demand
- Hot water used in the busiest hour, gallons
Worked examples
Family of four, three morning showers, gas
Three showers at 20 gal (60), a shave (2) and one round of food prep at the sink (3) add up to 65 gal in the peak hour — close to the 66 gal DOE worksheet example. A typical 40 gal gas heater with an FHR around 70 gal covers it.
Same morning, electric tank
Typical 40 and 50 gal electric tanks deliver only about 55 and 62 gal in the first hour, so the 65 gal peak calls for a 65 gal electric tank (FHR ≈ 75) — whereas a 40 gal gas tank would cover it.
Low-flow showers, electric tank
With short showers on low-flow heads using about 12 gal of hot water each, three showers take 36 gal; adding a shave (2) and food prep (3) gives 41 gal, which a typical 40 gal electric tank (FHR ≈ 55) handles.
Couple, two showers and a washer load, heat pump
Two showers (40), a shave (2) and a front-loader load (15) need 57 gal in the peak hour. Heat pump water heaters typically start at 50 gal, whose FHR of about 63 gal is enough.
Frequently asked questions
What size water heater do I need for a family of 4?+
Using the DOE worksheet, three back-to-back showers plus a shave and some sink use come to about 65 gallons, so many families of four need a first hour rating of roughly 60–70 gallons — typically a 40–50 gal gas tank or a 65 gal electric or heat pump tank. Calculate your own peak hour, since three back-to-back showers matter more than headcount.
What is a first hour rating?+
The first hour rating (FHR) is the number of gallons of hot water a storage water heater can supply in one hour, starting with a full tank of hot water. It is printed on the yellow EnergyGuide label of conventional storage heaters and in the product specifications, which is where to find it for heat pump models.
Why does a gas water heater need a smaller tank than electric?+
Gas burners (often 30,000–40,000 BTU/h) reheat water much faster than a 4,500 W electric element, so a gas tank refills with hot water quicker during heavy use and delivers more gallons in the first hour.
How many gallons of hot water does a shower use?+
The DOE peak-hour worksheet assumes about 20 gallons of hot water per shower. A short shower on a 1.5–1.8 gpm low-flow head uses much less — often 10–12 gallons of hot water — so adjust the per-shower figure to match your fixtures and habits.
Is a bigger water heater always better?+
No. An oversized tank costs more to buy and loses more heat standing idle. Choose the smallest model whose FHR covers your peak hour, with a little margin if your household is growing.