Pool Heater Size Calculator
Length × width for a rectangle.
82% is the US federal minimum.
US average: US average residential natural gas price.
Recommended heater size (input)
200k BTU/h
A 200,000 BTU/h gas heater covers the 164,839 BTU/h this pool needs (58.6 kW).
- Required input
- 164,839 BTU/h (48.3 kW)
- Heat-loss method (DOE)
- 135,168 BTU/h (39.6 kW)
- Heat-up method
- 118,150 BTU/h (34.6 kW) for 17 °F in 24 h
- One full heat-up costs
- $53.95 (natural gas)
- Target temperature
- 82 °F
How it's calculated
The calculator runs two methods and recommends a heater that satisfies the larger one.
- The heat-loss method is the one the U.S. Department of Energy gives on its Energy Saver page for gas swimming pool heaters. The factor 12 assumes a 1–1.25 °F per hour rise and an average wind of 3.5 mph across the water; DOE suggests multiplying by 1.5 for a 1.5 °F/h rise and by 2 for 2 °F/h.
- The heat-up method is basic physics: one BTU raises one pound of water by 1 °F, and a gallon weighs 8.34 lb.
- Efficiency: gas pool heaters sold in the US must reach at least 82% thermal efficiency (10 CFR 430.32). Heaters are sold by input rating, so the calculator divides the needed output by efficiency and rounds up to a common size.
- Fuel cost: 1 therm = 100,000 BTU and 1 gallon of propane ≈ 91,452 BTU (U.S. EIA). Default prices: natural gas $1.56/therm (U.S. EIA residential natural gas price series N3010US3; last checked 2026-10-29) and propane $2.73/gal (U.S. EIA Heating Oil and Propane Update). Replace them with your utility's rates.
Worked example
A 16 × 32 ft pool (512 sq ft, 20,000 gallons), currently 65 °F, wanted at 82 °F, coldest swim month averaging 60 °F, heat-up in 24 hours:
- DOE method: 512 × (82 − 60) × 12 = 135,168 BTU/h output.
- Heat-up method: 20,000 × 8.34 × 17 = 2,835,600 BTU ÷ 24 h = 118,150 BTU/h.
- Larger of the two: 135,168 BTU/h. Input at 82%: 135,168 ÷ 0.82 = 164,839 BTU/h.
- Next common size: 200,000 BTU/h.
- One heat-up with natural gas: 2,835,600 ÷ 0.82 ÷ 100,000 = 34.58 therms ≈ $53.95.
A solar or bubble cover is the cheapest way to make any heater go further: most heat leaves through evaporation at the surface, which a cover blocks.
Heater input by pool size and temperature rise
DOE method at 82% efficiency. "Rise" is desired water temperature minus average air temperature.
| Pool | Rise 10 °F | Rise 20 °F | Rise 30 °F |
|---|---|---|---|
| 12 × 24 ft | 42k | 84k | 126k |
| 15 × 30 ft | 66k | 132k | 198k |
| 16 × 32 ft | 75k | 150k | 225k |
| 18 × 36 ft | 95k | 190k | 284k |
| 20 × 40 ft | 117k | 234k | 351k |
Values in BTU/h of heater input (k = thousand).
Frequently asked questions
What size heater do I need for a 20,000 gallon pool?
For a typical 16 × 32 ft, 20,000 gallon pool heated to 82 °F when the coldest swim month averages 60 °F, the DOE method calls for about 135,168 BTU/h of output, or 164,839 BTU/h of input at 82% efficiency. That points to a 200,000 BTU/h heater.
How long does it take to heat a pool?
Divide the heat needed by the heater's output. Warming 20,000 gallons by 17 °F takes 2,835,600 BTU; a 200,000 BTU/h heater at 82% efficiency delivers 164,000 BTU/h, so about 17 hours before heat losses. Wind, evaporation and an uncovered surface can easily add 30–50% or more.
How much does it cost to heat a pool with gas?
One full heat-up of 20,000 gallons by 17 °F uses about 35 therms, roughly $53.95 at $1.56 per therm. Keeping it warm afterwards costs more over a season, mostly to replace heat lost from the surface, which a cover can cut sharply.
Is a bigger pool heater better?
Within reason, yes. Gas heaters cost about the same to run per BTU delivered regardless of size, so a larger heater simply reaches temperature faster. The trade-off is purchase price and gas line capacity, which must be sized for the heater's input rating.
Gas heater or heat pump?
Gas heaters heat fast and work in any weather, making them good for occasional use and short heat-ups. Heat pumps cost more upfront but usually far less to run when air temperatures are mild, so they suit pools kept warm all season.
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