Water Heater Size Calculator: Tank Gallons, First-Hour Rating or Tankless GPM

How it works

peak-hour demand = people × gal per person, or Σ uses × gal per use (DOE) · needed first-hour rating ≥ peak-hour demand · typical first-hour rating ≈ a × tank gal + b × 67 ÷ rise (electric 0.7, 28 · gas 0.7, 40 · heat pump 0.7, 30) · tankless: Btu/h = gpm × 500 × rise (°F), kW = Btu/h ÷ 3,412

A storage water heater is sized to your busiest hour, not to the day’s total. The US Department of Energy method adds up the hot water used in that hour (about 20 gal per shower, 2 per shave, 3 for hand dishwashing, 7 for a dishwasher, 25 for a top-loading or 15 for a front-loading washer) and asks for a first-hour rating at least that large. The first-hour rating, printed on the EnergyGuide label, is how much hot water a full tank delivers in one hour while it reheats, so it depends on the burner or element as well as the tank: a 40-gallon gas tank often beats a 50-gallon electric one. The quick mode multiplies the household by 10, 12 or 15 gal per person in the busiest hour. The calculator then estimates the first-hour rating of standard tank sizes for electric (4.5 kW), gas (about 40,000 Btu/h) and heat pump water heaters, scales the recovery to your temperature rise, picks the smallest tank that meets the demand and never goes below the common retailer rule of thumb for the household size. For a tankless heater the size is a flow rate: add the gallons per minute of everything that runs at once and multiply by 500 and the temperature rise to get the heat output in Btu per hour. Use the coldest incoming water of the year; DOE suggests 50 °F if you do not know it.

Worked example

A family of 4 with average use (12 gal per person in the busiest hour) needs 4 × 12 = 48 gal, so the heater’s first-hour rating should be at least 48 gal/hr. With 50 °F water heated to 120 °F (a 70 °F rise), a 4.5 kW electric tank delivers about 0.7 × tank + 26.8 gal/hr: a 40-gallon tank (≈ 55 gal/hr) already meets the demand, but the rule of thumb for 4 people is 50 gallons, so the recommendation is a 50-gallon (189 L) tank with an estimated first-hour rating of about 62 gal/hr. For a tankless gas heater running one 2.5 gpm shower and one 0.75 gpm faucet at the same rise: 3.25 gpm × 500 × 70 = 113,750 Btu/h of output, or about 119,737 Btu/h of gas input for a 95% condensing unit.

Sources: US DOE Energy Saver: Sizing a new water heater · ENERGY STAR residential water heaters key product criteria (first-hour rating, UEF) · Lowe’s water heater buying guide (capacity by household size) · Rheem Professional Classic electric water heater spec sheet (first-hour rating and recovery, 4.5 kW)

Frequently asked questions