Water heater size calculator

How water heater size is calculated

A tank water heater runs out when your busiest hour uses more hot water than the heater can deliver in that hour. The DOE method adds up that hour and compares it with the heater's first hour rating (FHR).

A tankless heater has no stored water, so it has to heat everything flowing at the same moment. Size it by flow and by how far it has to raise the water temperature.

The 60 × 8.34 part is 500.4, which is why installers round it to "GPM × 500 × rise".

Worked example: 80 gallon peak hour

The calculator opens on a household's busiest hour: 2 showers, 1 shave, 2 hand dishwashing or food-prep uses, 1 dishwasher load and 1 clothes washer load, with 2 showers and 1 faucet running at once and 55°F water coming in.

1. Peak hour: 2 × 20 + 1 × 2 + 2 × 3 + 1 × 7 + 1 × 25 = 80 gallons, so look for an FHR of at least 80.
2. That falls in the 61–80 gallon band: typically a 50 gal gas or 80 gal electric.
3. Tankless flow: 2 × 2.5 + 1 × 1.5 = 6.5 GPM. Rise: 120 − 55 = 65°F.
4. Output: 6.5 × 60 × 8.34 × 65 = 211,419 BTU/h, or 62.0 kW.

That is more than one residential gas tankless unit delivers (199,000 BTU/h) and far beyond a practical electric unit, so this household would either stagger showers, install two tankless units or stay with a tank.

Peak-hour gallons and tank sizes

Use these DOE figures to estimate your busiest hour, usually a weekday morning. Count only what really happens in the same hour.

The calculator maps the FHR you need to a typical tank. The first hour rating varies by model, so treat the tank as a starting point and confirm the FHR on the label.

Tankless sizing tips

• Use your local ground-water temperature: about 40°F in the north, 55°F in the middle of the country and 70°F in the south. Size for winter, when the inlet is coldest. If you don't know it, the DOE suggests assuming 50°F.
• Count only fixtures that really run together. A second shower adds 2.5 GPM; at a 65°F rise that is 81,315 BTU/h on its own.
• An electric tankless unit above about 36 kW (122,832 BTU/h) usually needs 150 A or more at 240 V. Check the panel and service before you buy.
• A gas unit needs the right gas pipe size and venting for its full input. The manufacturer's installation manual sets both.
• Tankless flow ratings drop as the rise goes up. Read the GPM the unit delivers at your rise, not the headline number.

Hire a licensed plumber for gas lines, venting and relief valves, and an electrician for any new circuit; many areas also require a permit to replace a water heater.