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Pool Heater Size Calculator
Quick answer: Size a pool heater two ways and take the larger: heat-up output equals gallons × 8.34 × degrees of rise ÷ hours you allow, and holding output is roughly surface area × (water minus air temperature) × 12. Divide the larger by efficiency to get the input rating to compare against spec sheets.
How it works
One BTU raises one pound of water by 1°F, and a US gallon weighs about 8.34 lb, so warming the whole pool takes gallons × 8.34 × temperature rise. Spread over the hours you are willing to wait, that is the output needed for the first heat-up. Once warm, the heater only has to replace losses, which happen mostly at the surface through evaporation, convection and radiation. A long-used industry rule puts that at about 12 BTU per hour per square foot per °F of difference between water and air in light wind; a nightly cover can roughly halve it, which the cover setting reflects. Gas heaters are labelled by input BTU/h, so the tool divides output by efficiency. For heat pumps, use the pool heat pump size calculator instead. Not sure of your gallons? Start with the pool volume calculator.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 20,000 gal, 512 sq ft, 65°F to 82°F in 24 h, 60°F nights, no cover, 84% efficiency. Heat-up output = 20,000 × 8.34 × 17 ÷ 24 ≈ 118,150 BTU/h. Holding = 512 × 22 × 12 ≈ 135,170 BTU/h. Input ≈ 135,170 ÷ 0.84 ≈ 161,000 BTU/h, so a pro might discuss models above that.
FAQ
Is bigger always better?
A larger gas heater warms faster and recovers quicker; the running cost per degree is similar. Gas line and venting limits often decide the size.
What about wind?
Wind can raise surface loss well above the rule of thumb. A cover is the cheapest fix.
Who confirms the size?
A licensed HVAC or pool professional, who also checks gas supply, venting and code.
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