The Department of Energy method
The U.S. Department of Energy sizes pool heaters from the surface area and the temperature rise, because most of a pool’s heat leaves through the surface. Its formula, used for both gas heaters and heat pumps, gives the heater output you need in Btu per hour:
Output (Btu/h) = surface area (ft²) × temperature rise (°F) × 12
Temperature rise = the water temperature you want minus the average air temperature in the coldest month you swim. The factor of 12 assumes the pool warms by 1 to 1¼°F an hour with about 3½ mph of wind. For 1½°F an hour multiply by 1.5; for 2°F, by 2.The DOE adds that windy, dry or cool-night locations need a bigger heater, and recommends a professional confirm the final size. Gas pool heaters range from 75,000 to 450,000 Btu/h.
Worked example
A 16 × 32 ft pool kept at 82°F, coldest swim month averaging 62°F
- Surface area = 512 ft²
- Temperature rise = 82 − 62 = 20°F
- Output = 512 × 20 × 12 = 122,880 Btu/h (36 kW)
- Gas heaters are sold by input. At 84% efficiency that’s 146k Btu/h input, so a 150k heater
- Heating 15,000 gallons from 65 to 82°F takes 2.13 million Btu, about 17 hours at that output
Heater output by pool size
| Surface area | 10°F rise | 15°F rise | 20°F rise | 25°F rise |
|---|---|---|---|---|
| 300 ft² | 36k | 54k | 72k | 90k |
| 400 ft² | 48k | 72k | 96k | 120k |
| 500 ft² | 60k | 90k | 120k | 150k |
| 600 ft² | 72k | 108k | 144k | 180k |
| 700 ft² | 84k | 126k | 168k | 210k |
| 800 ft² | 96k | 144k | 192k | 240k |
Heater output needed (thousand Btu per hour)
Gas heater or heat pump?
- Gas and propane heaters heat quickly in any weather. Modern units reach 89 to 95% efficiency, according to the DOE, so most of the fuel becomes heat in the water.
- Heat pumps move heat from the air into the water. The DOE says their coefficient of performance (COP) usually runs from 3 to 7, meaning 3 to 7 units of heat per unit of electricity. They cost more up front but much less to run, and they slow down when the air is cold.
Heat pumps are rated in output Btu at standard test conditions: 80°F air, 63% humidity and 80°F water under the federal test. Real output falls as the air cools, so size a heat pump with some margin if you swim in cool months.
Before you size up, consider a cover. Evaporation is the largest heat loss for most pools, and the DOE puts the heating savings from a cover at 50 to 70%. The pool heating cost calculator compares what each heater type costs to run.
Frequently asked questions
What size heater do I need for my pool?
Multiply the surface area by the temperature rise and by 12. A 512 ft² pool kept 20°F above the coldest month’s air needs about 123k Btu/h of output.
What size heat pump do I need for a 15,000 gallon pool?
Size it by surface area, not gallons. For a typical 16 × 32 ft pool, the DOE formula gives roughly 90k to 155k Btu/h of output for a 15 to 25°F rise. Add margin if you swim in cool weather.
How long does it take to heat a pool?
Divide the heat needed by the heater’s output. Raising 15,000 gallons by 17°F takes about 2.1 million Btu, so roughly 6 hours with a 400k input gas heater at 84% efficiency, before heat losses.
Is a bigger pool heater better?
A bigger gas heater warms the pool faster but costs more up front. Running costs depend mostly on how much heat the pool loses, not the heater size. Undersizing is the more common mistake.
What’s the difference between input and output Btu?
Gas heaters are labelled with input, the fuel they burn. Output is input times efficiency. The DOE formula gives output.
Does a solar cover reduce the heater size I need?
It reduces how much the heater has to run, and the DOE puts the savings at 50 to 70%. Size the heater for the times the cover is off.
Sources
- Gas Pool Heaters. U.S. Department of Energy, Energy Saver.
- Heat Pump Swimming Pool Heaters. U.S. Department of Energy, Energy Saver.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.