Pool Heat Pump Sizing Calculator

Sizing a pool heat pump

A pool heat pump sizing calculator gives the heat output you need to raise the water by a chosen number of degrees. Enter the pool surface area in square feet and the temperature rise in degrees Fahrenheit. The tool multiplies area, rise, and a factor of 12 to give BTU per hour.

Picture the water as a bathtub with the lid off. Heat escapes from the top. The more surface the water has, the faster it leaks away, so a bigger surface needs a bigger heater to keep up.

The formula in plain words

1. Surface area = length × width for a rectangle, or π × radius × radius for a round pool.
2. BTU per hour = surface area × temperature rise × 12.

The 12 is a rule-of-thumb factor for heater sizing, not a measured constant for your pool. It builds in a typical allowance for heat loss from the surface.

Worked example: a mid-size backyard pool

A pool is 15 ft by 30 ft, which is 450 sq ft. You want to lift the water 20°F.

• Surface area: 15 × 30 = 450 sq ft
• Temperature rise: 20°F
• Output: 450 × 20 × 12 = 108,000 BTU/hr

Worked example: a bigger pool with a smaller rise

A pool has 800 sq ft of surface, and you only need a 15°F lift.

• Output: 800 × 15 × 12 = 144,000 BTU/hr

Dropping the rise to 10°F on the first pool would give 450 × 10 × 12 = 54,000 BTU/hr, which shows how much the target temperature matters.

Output needed by pool size and rise

Surface area	10°F rise	15°F rise	20°F rise
200 sq ft	24,000	36,000	48,000
300 sq ft	36,000	54,000	72,000
450 sq ft	54,000	81,000	108,000
600 sq ft	72,000	108,000	144,000

All values are in BTU per hour. Cut the rise in half and the heater needed is cut in half.

Choosing the temperature rise

The rise is the gap between the coldest water you will see and the water you want. If your pool is 62°F at opening and you want 82°F, the rise is 20°F. If you only want to extend the season by a few weeks, a smaller rise may be enough.

Ask yourself how fast you want it to warm up. A larger heater warms the water sooner. A smaller one gets there too, just more slowly, and it may struggle in a cold snap.

Heat pump specifics

The formula above is the heat you need to deliver. A heat pump moves heat from the air into the water, so its output depends on how warm and humid the air is. Spec sheets list output at set air and water temperatures. Look at the rating near the conditions you will actually heat in, not only the headline number.

Gas heaters and electric resistance heaters are rated differently, so ask your installer for apples-to-apples numbers. A heat pump also needs an electrical circuit sized by an electrician, and its install should follow the manufacturer’s instructions and local code.

How to measure

For a rectangle, multiply the length by the width at the waterline. For a round pool, find the radius, then multiply π by the radius twice. For a freeform shape, split it into shapes you can measure and add them. Use the water surface, not the outside of the deck.

Common mistakes

• Using the pool’s volume instead of its surface. That produces a heater far too big or too small.
• Picking the rise from the target alone. The starting temperature matters just as much.
• Reading BTU ratings from a gas heater as if they apply to a heat pump. The output of a heat pump varies with the weather.
• Forgetting wind and shade. An exposed, windy pool loses more heat than a sheltered one, and the calculator does not model that.

Related calculators

Find the pool’s water volume with the pool gallons calculator and compare lifetime costs with the pool cost calculator. If you want to convert the BTU figure into other units, the BTU calculator can help.