Pool Heater Sizing: BTU Formula, Examples & Corrections

Quick answer

For an uncovered outdoor pool, the industry sizing rule is this: surface area (sq ft) × temperature rise (°F) × 12 = output BTU/hr to heat the pool in about 24 hours. A 16×32 pool (512 sq ft) heated 15°F needs ≈ 92,000 BTU/hr of output; buy a gas heater rated ≈ 112,000 BTU/hr input because gas is sold by fuel burned, not heat delivered. The BTU calculator applies the wind, shade and cover corrections and does the gas conversion automatically.

The sizing formula, step by step

1. Measure the surface. Rectangle: length × width. Round: π × radius². The surface is where evaporation — the largest heat loss in any outdoor pool — happens, which is why sizing runs on square footage rather than gallons.
2. Choose the temperature rise. Subtract your typical unheated water temperature from the temperature you want to swim at. Most owners target 78–82°F; a pool that sits at 60°F in May and a target of 80°F is a 20°F rise.
3. Apply the ×12 rule. Multiply area × rise × 12. The 12 encodes roughly 12 BTU/hr of heat escaping per square foot per degree of rise over a 24-hour heat-up, with normal wind and no cover. It is a sizing convention used across heater dealers and manufacturer sizing charts, not a law of physics — which is exactly why the corrections below exist.

Worked examples

Round to the next common size: gas heaters cluster at 150k, 200k, 250k, 300k, 333k and 400k BTU/hr input. A 16×32 pool with a 15°F rise lands comfortably on a 150k input unit; the 20×40 with a 20°F rise needs a 250k or better. The BTU quick table on the homepage generalizes these numbers.

Corrections: wind, shade and the solar cover

The ×12 baseline assumes a typical backyard. Real sites differ:

• Wind strips the warm, humid boundary layer off the water surface and accelerates evaporation. An exposed, windy site can add 20%; a fenced, hedged yard buys you roughly 10% back.
• Shade removes solar gain, so a shaded pool needs about 10% more heater than a full-sun pool.
• A solar cover is the single biggest lever you control. The DOE (via EPA WaterSense) credits pool covers with cutting evaporation by 30–50%, and evaporation is the dominant loss — so covering the pool meaningfully shrinks the heater you need. In sizing terms the calculator applies a conservative ×0.75; run the numbers with and without a cover and you'll see why cover-first is the cheapest upgrade in pool heating.

Gas input vs output: the 82% rule

Gas and propane heaters are rated by fuel input. Burners lose heat up the flue, so only a fraction reaches the water. Federal law sets the floor at 82% thermal efficiency for gas-fired pool heaters manufactured on or after April 16, 2013 (10 CFR 430.32), which is why a "400,000 BTU" gas heater delivers roughly 328,000 BTU/hr of actual heat. Convert before you compare: required output ÷ 0.82 = input rating to shop for.

Heat pumps and electric resistance heaters don't have this problem — they are rated by heat output, so their spec-sheet number compares directly with the calculator result. What heat pumps have instead is an air-temperature dependency, next section.

Heat pumps: why cold air changes the math

A heat pump doesn't create heat, it moves it from the air, so its delivered BTU/hr falls as the air cools. Manufacturer COP ratings (typically 5–6 at 80°F air) assume warm-air conditions; the IECC low-air rating floor is COP 4.0, and real-world output keeps dropping below ~55°F ambient. Practical consequences: size a heat pump one step up from the raw number if you swim in shoulder seasons, and don't expect heat-pump economics in a cold climate — the heat pump vs gas calculator prices the trade-off with your own rates.

How much margin to add

• Heat it faster than 24 hours? The ×12 rule is a 24-hour figure. Halving the window roughly doubles the required BTU/hr — the calculator handles the conversion for 48/72-hour windows too.
• One step up is normal. Undersized heaters run around the clock in cold snaps and never quite arrive; stepping up one common size costs little at purchase and buys headroom.
• Don't stack every correction twice. Corrections multiply (×1.2 wind × 1.1 shade), but they're estimates, not measurements — when in doubt, let the common-size rounding absorb the difference rather than padding each factor.

Where to go next

Run your exact pool through the BTU calculator, check how long heating will take with a specific model, estimate what it costs to run, or compare heat pump vs gas on the same pool. Every number on this page traces to the sourced formula above — nothing here is brand marketing.