Pool Heater Calculator
Heater size for your pool in BTU/hr or kW from surface area and the temperature rise you want.
What it does
A pool heater must replace the heat the water loses and still raise the temperature when asked. The quick sizing rule from the Department of Energy multiplies surface area by the temperature rise and a rate factor to get the Btu/hr a heater should deliver.
The rate encodes how fast you want the pool to warm. Rate 12 matches a gentle 1 to 1.25 F per hour, 18 about 1.5 F per hour, and 24 about 2 F per hour. Bigger rate, bigger target number, faster warm up.
Sizing lands on standard equipment: heat pumps come in 75,000, 100,000, and 125,000 Btu/hr sizes, and gas heaters step in 50,000 Btu/hr increments up to about 450,000. The tool picks the smallest step that covers the requirement.
Wind, humidity, night temperatures, and whether the pool wears a cover all move the real load, so treat the result as a starting size and adjust from there.
How to use
- Pick imperial (feet and F) or metric (meters and C).
- Enter the pool length and width at the water surface.
- Enter the temperature rise you want above ambient.
- Pick the heat up rate: 12, 18, or 24.
- Read the required heater output.
- Read the surface area and the recommended standard size.
Formula
- area
- Water surface area in square feet (or square meters converted).
- rise
- Degrees above ambient you want the water, F or C.
- 12
- Rate factor for about 1 to 1.25 F per hour of warm up.
- 18
- Rate factor for about 1.5 F per hour.
- 24
- Rate factor for about 2 F per hour.
The rule assumes a 3.5 mph wind over an outdoor pool. It ignores solar gain, covers, humidity, and the heater's own efficiency: a gas heater near 84 % turns fuel into heat better than the raw number implies, and a heat pump moves heat instead of making it, so its electrical draw is far below the Btu/hr rating.
Worked example
Example: 20 x 40 ft pool, 10 F rise
Length 20 ft, width 40 ft, rise 10 F, rate 12.
- Area: 20 x 40 = 800 ft2.
- Load: 800 x 10 x 12 = 96,000 Btu/hr.
- Recommended: 100,000 Btu/hr, the 5 hp heat pump class.
- Faster: the same pool at rate 18 needs 144,000 Btu/hr, a 150,000 gas step.
- Metric mirror: 6.1 m by 12.2 m is 74.4 m2, and a 5.6 C rise gives 28.4 kW.
- Tool shows 96,000 BTU/hr, 800 ft2, and the 100,000 BTU/hr recommendation.
Reference table
| Rate | Warm up pace | Required | Recommended |
|---|---|---|---|
| 12 | About 1 to 1.25 F per hour | 96,000 BTU/hr | 100,000 BTU/hr (5 hp heat pump) |
| 18 | About 1.5 F per hour | 144,000 BTU/hr | 150,000 BTU/hr (gas heater step) |
| 24 | About 2 F per hour | 192,000 BTU/hr | 200,000 BTU/hr (gas heater step) |
Limits
- Outdoor pool rule of thumb with a 3.5 mph wind; indoor pools run different numbers.
- Covers, wind, and humidity shift the real load by a wide margin.
- No fuel cost, efficiency, or heat pump coefficient of performance is computed.
- Altitude and swim season length are out of scope.
- Recommended sizes follow US equipment classes; other markets vary.
Common mistakes
- Sizing by pool gallons; the load tracks surface area.
- Reading the required number as the equipment size instead of stepping up to the next standard unit.
- Raising the rise input to heat faster when the rate setting already does that.
- Ignoring a cover; an uncovered warm pool at night can lose the whole gain.
- Switching to metric but retyping feet values.
Try it yourself
Quick checks. Answers sit tucked under each question.
16 x 32 ft pool, 15 F rise, rate 12?
512 ft2 x 15 x 12 = 92,160 Btu/hr, which lands on the 100,000 Btu/hr heat pump step.
Metric: 5 m by 10 m, 8 C rise, rate 18?
50 m2 with a 14.4 F equivalent rise gives 40.9 kW, recommended at 44 kW in the gas heater class.
Default metric pool 6.1 x 12.2 m, 5.6 C rise, rate 12?
74.4 m2 gives 28.4 kW, and the recommendation reads 29.3 kW in the heat pump class.
Questions
What size heater does my pool need?
Start with surface area x rise x 12 Btu/hr for a gentle warm up. A 20 x 40 pool rising 10 F lands at 96,000 Btu/hr, the 100,000 Btu/hr heat pump class.
Heat pump or gas?
Heat pumps cost less to run in mild air and warm slowly; gas heaters cost more per hour but heat fast and work in cold air. Size both from the same Btu/hr number.
What rise should I enter?
The gap between current water temperature and your target. Many owners plan 10 to 15 F over ambient.
Does a pool cover change the size?
A cover cuts losses dramatically, often letting a smaller heater hold temperature, though the initial warm up still follows this math.
Why did the tool pick a bigger number than required?
Equipment comes in standard sizes, so it steps up to the next class, like 150,000 Btu/hr gas, because 144,000 is not a shelf size.
Does the wattage of a heat pump match its output?
No. A heat pump rated 100,000 Btu/hr moves heat; its electrical draw is several times smaller. Compare units by rating, not by power draw.
Sources
- US Department of Energy: Heat Pump Swimming Pool Heaters
- US Department of Energy: Gas Swimming Pool Heaters