Water Heater Recovery Rate Calculator
A water heater's recovery rate is how many gallons it can heat to the target temperature in one hour. It depends on the heater's energy input, its efficiency, and the temperature rise from incoming cold water to the setpoint. This calculator uses the standard water-heating energy balance to compute recovery in gallons per hour for both gas (BTU) and electric (watt) heaters.
Recovery rate formula
Temperature rise = hot temp - cold temp
Total input BTU/hr = gas BTU + (watts * 3.412)
Net heat = input BTU * (efficiency / 100)
Recovery (gal/hr) = net heat / (8.33 * temperature rise)
8.33 = BTU to raise one gallon of water 1 degree F
It takes about 8.33 BTU to raise one US gallon of water by one degree Fahrenheit (one gallon weighs about 8.33 lb, and water's specific heat is 1 BTU per lb per degree F). One watt equals 3.412 BTU per hour. Efficiency accounts for heat lost up the flue or to standby.
Worked example
A 40,000 BTU per hour gas heater at 80 percent efficiency raising water from 50 to 120 F: rise = 70 F. Net heat = 40,000 times 0.80 = 32,000 BTU per hour. Recovery = 32,000 / (8.33 times 70) = 32,000 / 583.1 = 54.88 gallons per hour. In 30 minutes it recovers about 27.44 gallons.
Recovery rate notes
- Recovery rate matters most for households with high simultaneous hot-water demand.
- Electric resistance heaters are nearly 100 percent efficient at the element but have lower BTU input, so recovery is slower than most gas units.
- Lower incoming water temperature in winter increases the temperature rise and slows recovery.
- First-hour rating combines recovery with stored tank volume; this tool computes recovery only.
- 8.33 BTU per gallon per degree F is the fundamental water-heating constant.
Water Heater Recovery Rate Calculator: frequently asked questions
What is water heater recovery rate?
Recovery rate is the number of gallons a water heater can raise to the set temperature in one hour. It depends on the energy input, the heater's efficiency, and how much the water temperature must rise from incoming cold to the setpoint.
Why is gas recovery faster than electric?
A typical gas water heater inputs 30,000 to 50,000 BTU per hour, while a standard 4,500 watt electric element inputs only about 15,400 BTU per hour. Even though the electric element is nearly fully efficient, its lower input means it heats fewer gallons per hour.
How does cold incoming water affect recovery?
Recovery rate is inversely proportional to the temperature rise. In winter, incoming water can be much colder, increasing the rise and reducing the gallons per hour the heater can deliver at the same setpoint.
What is the 8.33 number in the formula?
One US gallon of water weighs about 8.33 pounds, and it takes 1 BTU to raise 1 pound of water by 1 degree Fahrenheit. So heating one gallon by one degree takes about 8.33 BTU. That constant links energy input to gallons heated.
Sources and methodology
- U.S. Department of Energy: Water Heating.
- The energy balance (8.33 BTU per gallon per degree F, 3.412 BTU per watt-hour) uses fixed physical constants; efficiency and temperatures are user inputs.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.