What size water heater for a family of 2, 4 or 6?

Water heater size by household: 1 to 6 people

A tank water heater is sized by its first hour rating (FHR): the gallons of hot water it delivers in one hour starting full. The U.S. Department of Energy says to pick a heater whose FHR at least matches your peak-hour demand, the hot water used in your busiest hour. Head count matters only because it sets what happens in that hour.

The table assumes a busy weekday morning: every person showers once (20 gal each), up to 2 adults shave (2 gal each), and there is 1 round of hand dishwashing or food prep (3 gal). No dishwasher or washing machine runs in that hour. The last two columns move one shower to the evening.

Each shower that moves out of the busiest hour lowers the FHR you need by 20 gallons. That one habit is often the difference between tank sizes, which is why the same family of four can be well served by a 50 gal gas or 80 gal electric or need a 75 gal gas or high-input 50 gal gas.

Adjust the peak hour for your household

The profile above is a starting point, not a rule. Change it to match the hour your household actually uses the most hot water, usually a weekday morning or the evening bath-and-dishes hour.

• Laundry: a top-loading washer load in the same hour adds 25 gallons (a front-loader about 15). A family of four with a top-loader running during showers needs an FHR of 112: a 100 gal tank, two heaters or a tankless system.
• Dishwasher: a load adds 7 gallons. Running it at night keeps it out of the morning peak.
• Baths: count a bath like a shower, 20 gallons, or more for a large tub.
• Young children: if they bathe at night, count only the people who shower in the morning.
• Long showers: the 20 gallon figure is an average. A household that takes long showers, or has a shower with more than one head, should count more per shower.

Tankless flow for a family in winter

A tankless heater has no stored water, so household size matters through the fixtures that run at the same moment, and the coldest inlet water of the year. The table sizes for a 40°F winter inlet, an 80°F rise to 120°F, with showers at 2.5 GPM and faucets at 1.5 GPM.

The DOE says a gas tankless unit typically manages a 70°F rise at about 5 GPM and an electric one at about 2 GPM. At an 80°F winter rise the same output gives only about 4.4 and 1.8 GPM. Above about 36 kW an electric whole-house unit is impractical, which rules it out from 2 people up in this table. Past 199,000 BTU/h a single residential gas unit is not enough, which applies from 3–4 people up.

Reading the first hour rating on the label

The FHR is printed in the top left corner of the yellow EnergyGuide label as "Capacity (first hour rating)". Tank gallons alone do not tell you much: a gas tank recovers faster than an electric one of the same size. Typical 50 gal gas tanks rate about 80–90 gallons and 50 gal electric tanks about 60–65, so compare the FHR from your table row with the label, not the tank size.

Replacing a water heater usually needs a permit. Gas venting, gas pipe sizing, relief valves and any new electrical circuit follow the manufacturer's instructions and local code.