The short version
Heat pumps are sized on surface area, not gallons. The formula is: BTU per hour = surface area in square feet, times the temperature rise you want in Fahrenheit, times a factor between 6 and 12 depending on how fast you want to get there.
For most people with a 24 ft round pool in a temperate climate, that lands around 55,000 BTU. The table further down gives every common size.
Two things matter more than getting the number exactly right. Buy a cover, which halves your heat loss and is the cheapest heating upgrade in existence. And accept that a heat pump warms a pool over days, not hours — if you want same-afternoon heat, you want gas, and you will pay for it every month.
Why surface area, not gallons
Nearly all the heat a pool loses goes out through the surface, through evaporation and radiation to the night sky. Very little escapes through the walls or floor. Evaporation alone typically accounts for around 70 percent of the loss.
This is why volume is the wrong basis for sizing. A wide shallow pool and a narrow deep pool holding identical water lose heat at very different rates, because one presents far more surface to the air. A heat pump has to replace what is lost, so it is sized against the surface.
Volume still matters, but only for one thing: how long the initial heat-up takes. Once the pool is at temperature, holding it there is purely a surface-area problem.
This also explains why a cover changes everything. A solar cover sits on the surface and largely stops evaporation, cutting total heat loss by 50 to 70 percent. That is a bigger effect than any realistic difference in heat pump size.
How much temperature rise do you need?
Temperature rise is the gap between what your pool sits at naturally and what you want. Most people target 80 to 84 degrees Fahrenheit. An unheated pool typically runs 10 to 20 degrees below air temperature during the shoulder seasons.
| Climate | Typical rise needed | What to expect |
|---|---|---|
| Warm (FL, TX, AZ) | 10 F | Extends the season either side of summer; the pool is already close to target. |
| Temperate (CA, mid-Atlantic) | 15 F | The most common case. Comfortable swimming from spring through autumn. |
| Cool (Pacific NW, Northeast) | 20 F | Size up, and expect the unit to lose efficiency as the air gets colder. |
BTU sizing by pool size
The figures below assume a 20 degree rise, which is the demanding case. Two columns are given because sizing charts quietly disagree, and the disagreement is worth understanding rather than hiding.
The professional figure sizes for a full heat-up in about a day. It is what a pool company will spec, and it is correct — but it produces numbers far larger than what is actually sold, which is why so many people conclude every heat pump on the market is undersized.
The realistic figure is what homeowners overwhelmingly buy. A smaller unit reaches exactly the same temperature; it just takes two or three days from cold, and then holds it without difficulty. For a pool you heat once in spring and leave running, that is usually the better buy.
Size up toward the professional column if you are in a cool climate, heat the pool from cold repeatedly rather than continuously, or do not intend to use a cover.
| Pool | Surface area | Realistic size | Pro spec | Heat-up from cold |
|---|---|---|---|---|
| 12 ft round | 113 sq ft | 15,000 BTU | 30,000 BTU | ~3 days |
| 15 ft round | 177 sq ft | 25,000 BTU | 45,000 BTU | ~3 days |
| 18 ft round | 254 sq ft | 35,000 BTU | 65,000 BTU | ~3 days |
| 21 ft round | 346 sq ft | 45,000 BTU | 85,000 BTU | ~3 days |
| 24 ft round | 452 sq ft | 55,000 BTU | 110,000 BTU | ~3 days |
| 27 ft round | 573 sq ft | 70,000 BTU | 140,000 BTU | ~3 days |
| 30 ft round | 707 sq ft | 85,000 BTU | 170,000 BTU | ~3 days |
| 33 ft round | 855 sq ft | 105,000 BTU | 210,000 BTU | ~3 days |
Working it out for your own pool
For a rectangular pool, surface area is simply length times width. For a round pool it is 3.14 divided by 4, times the diameter squared — a 24 ft round pool works out at about 452 square feet.
Then: area, times your temperature rise, times 6 for a realistic size or 12 for a fast one. A 452 square foot pool wanting a 20 degree rise gives 452 x 20 x 6, which is about 54,000 BTU. Round up to the nearest size sold, so 55,000 or 60,000.
Heat pumps come in coarse steps, so you will rarely hit your number exactly. Round up rather than down. The penalty for a slightly oversized heat pump is that it costs a bit more to buy; the penalty for an undersized one is that it runs constantly and never quite gets there.
Heat-up time and what it actually feels like
Raising one gallon of water by one degree takes about 8.34 BTU. So a 13,000 gallon pool needs roughly 2.2 million BTU to come up 20 degrees.
At 55,000 BTU per hour that is about 40 hours of running. But a heat pump only produces its rated output while the air is warm, so you get perhaps 12 useful hours a day rather than 24. In practice that is a three-day heat-up from cold.
This is the expectation gap that produces most heat pump complaints. Nothing is broken; the physics is simply slower than people assume. Once at temperature, holding it costs far less than getting there.
The practical approach is to heat once at the start of the season, leave the unit running on a thermostat, and use a cover overnight. Heating from cold every weekend is the expensive way to own a heat pump.
Running costs
Heat pumps move heat rather than making it, which is why they win on running cost. Their efficiency is quoted as COP, and a good unit sits between 5 and 6 in mild conditions — meaning about five units of heat for every unit of electricity. A gas heater cannot beat 1.
In real terms, expect roughly 100 to 250 dollars a month to heat a mid-size pool through the shoulder season, against 400 to 800 dollars for gas doing the same job. The heat pump costs more to buy and pays that back within a season or two of regular use.
COP falls as the air gets colder, which is the heat pump's real limitation. Below about 50 degrees Fahrenheit output drops sharply, and below 45 most units stop being worthwhile. If you want to swim in genuine cold, gas is the honest answer.
A cover remains the highest-return purchase in pool heating. Halving your heat loss halves your bill, and a solar cover costs a fraction of one month's heating.
Electrical requirements people miss
A heat pump is a large appliance and usually needs its own dedicated circuit. Small units around 50,000 BTU typically want a 30 to 40 amp 240 volt supply; larger ones want 50 amps or more.
Budget for an electrician. Running a new dedicated circuit to the pool equipment pad commonly costs 500 to 1,500 dollars, and it is not a place to improvise — this is permanent outdoor wiring next to water.
Check your panel has the capacity before you buy. Discovering you need a service upgrade after the heat pump arrives is a genuinely expensive surprise, and it happens often enough to be worth ten minutes with a flashlight.