Water Heater Size Calculator
A water heater size calculator adds your busiest hour of hot water use. 3 showers, a shave, and hand washing need a 65 gal first hour rating by the DOE method.
Water heater size calculator: match the busiest hour
A water heater size calculator adds up the hot water your home uses in its busiest hour. That total is the first hour rating (FHR) a tank heater needs. Using the Department of Energy worksheet values, 3 showers, 1 shave, and 1 round of hand dishwashing come to 60 + 2 + 3 = 65 gallons. Look for a heater with an FHR of at least 65.
The DOE method in plain words
Think of a bathtub that must stay full of hot water for one hour. You count every tap that will pull from it in that hour. You add up the gallons. The heater has to supply that many.
The worksheet gives one gallon figure per use:
| Use in the peak hour | Gallons per use |
|---|---|
| Shower | 20 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-loader | 25 |
| Clothes washer, front-loader (H-axis) | 15 |
First hour rating needed = sum of (uses × gallons per use). The DOE notes that the worksheet covers one hour and not total daily use.
Worked examples
These match the calculator exactly.
Example 1: the DOE worksheet household. 3 showers, 1 shave, 1 hand dishwashing, no dishwasher, no laundry.
- Showers: 3 × 20 = 60
- Shave: 1 × 2 = 2
- Hand dishwashing: 1 × 3 = 3
- First hour rating needed: 65 gal
The DOE page prints 66 gallons for this example. The three lines add to 65, and the calculator uses 65.
Example 2: a family of 4. 2 showers, 1 shave, 1 hand dishwashing, 1 dishwasher load, 1 front-loader laundry load.
- Showers: 2 × 20 = 40
- Shave: 1 × 2 = 2
- Hand dishwashing: 1 × 3 = 3
- Dishwasher: 1 × 7 = 7
- Washer: 1 × 15 = 15
- First hour rating needed: 67 gal
- Typical tank for 4 people: 40 to 50 gallons
- Tankless, 2 showers at once: 5.0 GPM at 2.5 gpm, or 4.0 GPM at 2.0 gpm
Tank size by household
| People | Typical tank (Rheem, gas) |
|---|---|
| 1 to 2 | 30 to 40 gallons |
| 3 to 4 | 40 to 50 gallons |
| 5 | 50 to 55 gallons |
| 6 or more | 75 gallons or a tankless unit |
A gas tank recovers faster than an electric one, so the same tank size gives a higher FHR on gas. Compare the FHR on the EnergyGuide label with your result.
The tankless note
A tankless heater has no stored water. It needs enough flow while the taps run. Add the gallons per minute of the fixtures that run together. A standard showerhead uses about 2.5 gpm and a WaterSense showerhead no more than 2.0 gpm. The heater must also lift the water temperature from the incoming water to your set point, so a cold-climate unit needs more heating power for the same flow.
How to measure
- Choose the busiest hour of a normal day, not the one holiday morning.
- Count showers and laundry loads that overlap in that hour.
- Check whether your washer is a top-loader or a front-loader.
- Count dishwasher loads that run in the hour.
Common mistakes
- Using total daily hot water. The method sizes for one hour.
- Matching tank gallons to the result. The result is a first hour rating, and tank gallons are a different number.
- Ignoring the washer type. A top-loader uses 25 gal and a front-loader 15.
- Sizing a tankless unit by household size. Size it by flow and temperature rise.
- Counting every shower in the house as simultaneous when only two ever overlap.
Sources
- US Department of Energy, Sizing a New Water Heater: the peak hour worksheet and per-use gallons (energy.gov/energysaver/sizing-new-water-heater).
- Rheem, What Size Gas Water Heater Do I Need for My Home: tank size by household (rheem.com).
- US EPA WaterSense, Showerheads: about 2.5 gpm for standard heads and 2.0 gpm or less for labeled models (epa.gov/watersense/showerheads).
Related calculators
For flow units, use the GPM calculator. To work out heating power, see the BTU calculator. A home with a septic system should also check the septic tank size calculator, since hot water use feeds it.
Frequently asked questions
What is a first hour rating?
The first hour rating (FHR) is the number of gallons a tank water heater can deliver in one hour, starting with a full tank of hot water. It is printed on the EnergyGuide label of tank water heaters sold in the US. It combines the stored hot water with how fast the burner or element heats more.
How do I find my peak hour demand?
Pick the hour of the day when your home uses the most hot water, usually a weekday morning. List the showers, shaves, dishes, and laundry loads in that hour and multiply each by the gallons the DOE worksheet gives. The total is your peak hour demand.
Should the first hour rating match the peak hour demand exactly?
The DOE says to look for a model with a first hour rating that at least matches your peak hour demand. A rating a little above is a safer buy than one below.
What tank size goes with each household?
Rheem suggests 30 to 40 gallons for 1 to 2 people, 40 to 50 gallons for 2 to 4 people, 50 to 55 gallons for 4 to 5 people, and 75 gallons or a tankless unit for larger homes. Tank size is a starting point. The first hour rating is the number to match.
How do I size a tankless water heater?
Tankless units are sized by flow in gallons per minute at your temperature rise, not by a tank. Add the flow of the fixtures that run together. Two showers at 2.5 gpm need 5 GPM. WaterSense showerheads use 2.0 gpm or less, so the same two showers need 4 GPM.
Estimates only. Verify quantities with your supplier before purchasing.