Water Heater Size Calculator: What Size Water Heater Do I Need?

The short answer

A family of four, with everyone showering in the busiest hour, needs a 40 gallon gas tank or a 50 gallon electric tank where the water comes in at 59°F. In Minneapolis, where it comes in at 50°F, that is a 40 gallon gas tank, 50 electric or 65 heat pump. A tankless unit for two showers and a faucet (5 gallons per minute) needs about 121,100 BTU/h in Houston (74°F water) but 184,200 BTU/h in Minneapolis (50°F).

The same household in every state

A 4-person household that all uses hot water in the busiest hour (61 gallons), and a tankless gas unit for two showers and a bathroom faucet (5 gpm), sized for the water temperature of each state's largest city.

How it works

The formula

For a tank, we match the first-hour rating (FHR) to the hot water you use in your busiest hour, the method in DOE Energy Saver's Sizing a new water heater. For a tankless heater, we match the flow you run at once and how much the water must be heated.

Assumptions

• Hot water used per event: a shower 10 gallons (2 gpm for 5 minutes), a bath 20, a dishwasher load 6, a clothes washer load 7, shaving or hand washing 2. These follow the DOE Energy Saver worksheet and are our rounded values for modern fixtures.
• Default busiest hour: everyone showers and washes up, and the house runs one dishwasher load (two or more people) and one laundry load (three or more).
• Typical FHR at the 58°F DOE test inlet, in gallons. Gas: 30 gal 55, 40 gal 70, 50 gal 82, 65 gal 95, 80 gal 105. Electric: 30 gal 42, 40 gal 55, 50 gal 68, 65 gal 80, 80 gal 92. Heat pump (hybrid mode): 50 gal 63, 65 gal 75, 80 gal 85. Real models differ, so check the EnergyGuide label.
• Colder inlet water lowers the FHR. About 70% of a tank is hot when you start; the rest of the FHR is recovery, which shrinks by (120 − 58) ÷ (120 − inlet). Tanks are set to 120°F.
• Tank sizes: 30, 40, 50, 65, 80 gallons. Heat pump tanks start at 50.
• Tankless flow per fixture (EPA WaterSense maximums or typical): shower 2 gpm, bathroom faucet 1, kitchen faucet 1.5, dishwasher 1.5, clothes washer 2. We count all of the flow as hot, which is on the safe side, because real showers mix in cold water.
• The 500 is 8.33 pounds per gallon × 60 minutes. Gas units are 95% efficient and electric 99%.
• Standard gas tankless sizes: 140k, 160k, 180k, 199k BTU/h. Electric: 18, 24, 27, 36 kW. If one isn't enough we show how many units you would need. A whole-house electric tankless often needs 120 to 160 amps.

Worked example: a family of 4 in Minneapolis

Too big or too small

• Too big (tank): wasted energy. A bigger tank keeps more water hot all day and loses more heat. It also costs more to buy and takes more floor space.
• Too small (tank): you run out. The last shower turns cold. Its first-hour rating is the number to watch, not its gallons.
• Too small (tankless): temperature drops. If more fixtures run than the unit can heat, the water comes out lukewarm. Running two showers at once is what usually does it.
• Too big (tankless): a larger unit needs a bigger gas line, a larger vent or heavier electrical service. All of it costs more up front.
• Between sizes? Take the larger tank if you have the room, or the larger tankless unit if you have a lot of fixtures. A heat pump water heater with an FHR close to the need works fine, because it can switch to a faster mode.