Pool Heater BTU Calculator
How many BTUs does your pool actually need? Enter your pool's dimensions and target temperature — the calculator applies the industry sizing formula with honest corrections for wind, shade and a solar cover, then converts to the rating you should shop for, whether you're buying gas, propane, electric or heat pump.
Pool heater BTU quick table
Output BTU/hr · 24-hour heat-up · uncovered poolCommon rectangular pools at 4.5 ft average depth. Gas heaters are sold by input — divide any figure by 0.82 (or just use the calculator, it does the conversion).
| Pool size | Volume | +10°F | +15°F | +20°F | +25°F | +30°F |
|---|---|---|---|---|---|---|
| 12×24 ft | 9,700 gal | 35,000 | 52,000 | 69,000 | 86,000 | 104,000 |
| 14×28 ft | 13,200 gal | 47,000 | 71,000 | 94,000 | 118,000 | 141,000 |
| 16×32 ft | 17,200 gal | 61,000 | 92,000 | 123,000 | 154,000 | 184,000 |
| 18×36 ft | 21,800 gal | 78,000 | 117,000 | 156,000 | 194,000 | 233,000 |
| 20×40 ft | 26,900 gal | 96,000 | 144,000 | 192,000 | 240,000 | 288,000 |
Figures are heater output BTU/hr. Windy site or no solar cover pushes the real number up — the calculator above applies those corrections.
How pool heater sizing works
Surface area
length × width (sq ft)
× Temp rise
target − now (°F)
× 12 rule
industry 24-hour sizing
Market size
150k–400k BTU/hr
Heat escapes at the surface
Evaporation is the largest heat loss in an outdoor pool, so sizing starts with surface area (sq ft), not gallons. Gallons matter for how long heating takes, not how big the heater must be.
The industry rule: × 12
Area × temperature rise (°F) × 12 = output BTU/hr to heat in about 24 hours, uncovered, typical wind. It assumes ~12 BTU/hr escapes per square foot per degree of rise.
Correct for reality
Wind adds loss, shade slows the sun, a solar cover cuts the biggest loss by 30–50% (DOE). Gas then divides by 0.82 — the federal minimum efficiency — to get the input rating on the box.
Works for every heater type
Gas & propane heaters
Fast heat-up, air-temperature proof, rated by fuel input — the federal 82% efficiency floor decides what actually reaches the water.
The 82% rule explained →Heat pumps
3–6× the heat per kWh of direct electric heat, but output falls in cold air — sizing margins and climate decide the payoff.
Heat pump vs gas →Electric resistance
Simple and 100% efficient at the tank, but every BTU is billed at kWh rates — realistic for small pools and spas only.
Estimate running cost →Heater brands, spec by spec
Brand datasheet pages: model-by-model BTU ratings, efficiency and COP from official sources — Hayward, Pentair, Raypak, Jandy and more, comparable across brands.
Compare heater brands →Not sure what size?
The full sizing guide walks the formula step by step: surface area, temperature rise, heat-up windows, covers and margins.
Read the sizing guide →
FAQ
How many BTUs does my pool heater need?
For an outdoor uncovered pool, the industry sizing rule is: surface area (sq ft) × temperature rise (°F) × 12 — that gives the output BTU/hour needed to heat the pool in about 24 hours. A 16×32 pool (512 sq ft) with a 15°F rise needs roughly 92,000 BTU/hr of output. Wind, shade and a solar cover move the number up or down, and gas heaters are rated by input rather than output — the calculator above applies every correction and converts for you.
Why are gas pool heaters rated by input BTU?
Gas burners lose some heat up the flue, so manufacturers rate them by fuel input, not heat delivered. Federal law (10 CFR 430.32) requires gas pool heaters to be at least 82% thermally efficient, so a "400,000 BTU" gas heater actually delivers roughly 328,000 BTU/hr into your pool. Heat pumps and electric resistance heaters are rated by heat output, so their rating can be compared directly with the calculator's result.
Is a bigger pool heater better?
Within reason, yes — an undersized heater runs around the clock and may never reach your target temperature in cooler months, while a correctly sized unit heats faster and then cycles off. Stepping a heat pump up one size also compensates for its output dropping in cold air. The practical ceiling is your gas line size, your electrical service and your budget — the calculator's "shop for" figure is the minimum to look for, and one common size up is a reasonable margin.
Does a solar cover really reduce the heater size I need?
Yes — evaporation is the largest heat loss in an outdoor pool, and the DOE (via EPA WaterSense) credits pool covers with cutting evaporation by 30–50%. In sizing terms that typically trims the required BTU figure meaningfully; the calculator shows your required BTU both with and without a cover so you can see the difference instead of taking it on faith.
Is this website affiliated with any heater brand?
No. This site is heater-brand neutral on purpose: brand calculators only ever run the math for their own product line, so the sizing number is not comparable across brands. Every formula, correction factor and efficiency figure here is sourced and dated on the page, and brand datasheet pages link the official manufacturer spec for every number.