Pool Heater Size Calculator (BTU for Gas Heaters and Heat Pumps)
Heater output in Btu per hour for an outdoor pool from surface area and temperature rise, using the U.S. Department of Energy formula, plus gas input, the matching standard heat pump size and the time to warm the water.
Formula
Temperature rise = desired pool temperature − average air temperature of the coldest month of use. Heater output (Btu/h) = surface area × rise × 12 × rate factor (1, 1.5 or 2). Gas input = output ÷ efficiency. Heat to warm the water once = gallons × 8.34 lb × (desired − starting temperature); hours = that ÷ output, with no losses.
Worked example
Inputs: Pool surface area (length × width for a rectangle) = 512 sq ft; Desired pool temperature = 80 °F; Average air temperature, coldest month you swim = 65 °F; Heat-up rate = 1 to 1¼ °F per hour (DOE base); Gas heater efficiency (nameplate) = 80 %; Pool volume (optional, for warm-up time) = 22000 gal; Starting water temperature = 70 °F.
| Temperature rise °F | 15 |
|---|---|
| Heater output Btu/h | 92,160 |
| Gas heater input Btu/h | 115,200 |
| Heat pump standard size | 5 hp / 100,000 Btu |
| Heat to warm water Btu | 1,834,800 |
| Hours to warm at output | 19.9 |
DOE approximation for an outdoor pool (1 to 1¼ °F per hour, 3½ mph wind at the surface). Wind, low humidity and cool nights need a larger heater; a pool cover cuts the load. Gas heaters are sold by input, so compare the input figure with the nameplate. Warm-up time ignores heat lost while heating.
FAQ
What size pool heater do I need?
Multiply surface area by the temperature rise and by 12. A 16 × 32 ft pool (512 sq ft) kept at 80 °F where the coldest swim month averages 65 °F needs 512 × 15 × 12 = 92,160 Btu/h of output. DOE lists standard heat pump sizes of 75,000, 100,000 and 125,000 Btu, so that pool lands on the 5 hp/100,000 Btu size.
Gas heater: output or input?
Gas heaters are labeled by input. Divide the output by the efficiency: 92,160 ÷ 0.80 = 115,200 Btu/h input. DOE notes some gas pool heaters now reach 89–95% efficiency.
What does the formula assume?
DOE says it is based on a 1 to 1¼ °F per hour rise and a 3½ mph average wind at the pool surface. Windier, drier sites with cool nights need a larger heater. DOE also recommends a professional sizing analysis.
How long will it take to heat my pool?
Warming 22,000 gallons by 10 °F takes 22,000 × 8.34 × 10 = 1,834,800 Btu; at 92,160 Btu/h that is about 20 hours before losses. A cover cuts the losses.
Does this work for spas or indoor pools?
No. The DOE formula is for outdoor pools. Spas and indoor pools have different heat losses; use the heater maker's sizing guide.
Sources
- U.S. Department of Energy, Energy Saver: Gas Swimming Pool Heaters and Heat Pump Swimming Pool Heaters (Pool Area × Temperature Rise × 12; based on 1 to 1¼ °F per hour and 3½ mph wind; ×1.5 for 1½ °F, ×2 for 2 °F; heat pump standard sizes 75,000/100,000/125,000 Btu). The energy.gov page now returns 404; this is a full copy of the DOE text hosted by a retailer
- USGS Circular 1268, conversion factors: 1 gallon of water = 8.34 pounds
Related
- Pool Volume Calculator: Rectangular (Gallons)
- Tankless Water Heater Size Calculator (GPM, Temperature Rise, BTU & kW)
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Last reviewed 2026-10-30.