Pool heater size calculator
To warm a 15,000-gallon pool by 15 °F in 24 hours, a heater must deliver at least 78,000 BTU per hour, before any heat loss. Outdoor pools need more.
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That is the heat delivered to the water. At 80% efficiency, the heater needs about 98,000 BTU/h of input. It is the minimum with zero heat loss.
About these numbers. This is heat-up physics (gallons × 8.34 × °F rise ÷ hours), not yet backed by the tested reference cases we use for chemistry. The 80% efficiency default is an example value; use the rating on your heater's nameplate or spec sheet.
Calculator: gallons, rise, hours
Output needed (BTU/h) = gallons × 8.34 × temperature rise ÷ hours to heat. Input rating = output ÷ efficiency. Gas heaters are usually rated by input, and heat pumps by output. The table shows how fast the requirement grows when you want heat sooner:
| Pool, rise, time | Delivered to water | Heater input at 80% |
|---|---|---|
| 10,000 gal, +15 °F, 24 h | 52,125 BTU/h | 65,156 BTU/h |
| 15,000 gal, +15 °F, 48 h | 39,094 BTU/h | 48,867 BTU/h |
| 15,000 gal, +15 °F, 24 h | 78,188 BTU/h | 97,734 BTU/h |
| 15,000 gal, +15 °F, 12 h | 156,375 BTU/h | 195,469 BTU/h |
| 20,000 gal, +20 °F, 24 h | 139,000 BTU/h | 173,750 BTU/h |
| 30,000 gal, +20 °F, 24 h | 208,500 BTU/h | 260,625 BTU/h |
Halving the heat-up time doubles the heater size you need. Not sure of your gallons? Start with the pool volume calculator.
Heat loss: quick estimates
Once the pool is warm, the heater's job is to keep up with losses through the surface, so professionals size by surface area, not gallons. The U.S. Department of Energy (DOE) gives this estimate for outdoor pools:
Heater output (BTU/h) = pool surface area (sq ft) × temperature rise (°F) × 12.
The rise here is your desired water temperature minus the average air temperature of the coldest month you swim. DOE says the formula assumes a 1 to 1¼ °F rise per hour and a 3½ mph average wind at the surface. Multiply by 1.5 for a 1½ °F-per-hour rise, and by 2.0 for 2 °F per hour.
| Pool (surface) | +10 °F | +15 °F | +20 °F |
|---|---|---|---|
| 12 × 24 ft (288 sq ft) | 34,560 | 51,840 | 69,120 |
| 24 ft round (452 sq ft) | 54,287 | 81,430 | 108,573 |
| 16 × 32 ft (512 sq ft) | 61,440 | 92,160 | 122,880 |
| 18 × 36 ft (648 sq ft) | 77,760 | 116,640 | 155,520 |
| 20 × 40 ft (800 sq ft) | 96,000 | 144,000 | 192,000 |
Output, not input. For a gas heater, divide by its efficiency to get the input rating to shop for.
Use both numbers: the gallons method tells you how long a heat-up will take, and the surface method tells you whether the heater can hold temperature on a cool, breezy day. DOE still recommends that a trained pool professional do the final sizing, since wind exposure, humidity, and cool nights all raise the size you need.
Gas vs heat pump sizing
- Gas heaters come in outputs from about 75,000 to 450,000 BTU (DOE). At the same efficiency, a larger unit burns about the same fuel per degree and heats the pool faster, which matters most if you heat only for weekends.
- Heat pumps commonly come as 3.5 hp/75,000 BTU, 5 hp/100,000 BTU, and 6 hp/125,000 BTU (DOE). Their output drops as the air cools, and DOE says efficiency falls when the air is below roughly 45–50 °F. Size for the coolest month you want to swim, not for July.
What will running it cost? The heater cost calculator turns BTU into therms, kWh, and dollars at your rates.
Cover effect
A cover changes the answer more than any heater choice. Evaporation is the biggest heat loss from a pool, and the DOE says covering a pool when it is not in use can save 50–70% of heating costs. With a cover on overnight, a smaller heater can hold the same temperature, and the heat-up hours in the first table get closer to real life. Without one, even a heater sized by the surface method may not keep up on windy nights.
FAQ
What size heater do I need for my pool?
Check two numbers. For heat-up speed: gallons × 8.34 × °F rise ÷ hours (15,000 gallons, +15 °F in 24 hours needs about 78,000 BTU/h delivered). For holding temperature: surface area × rise × 12 (DOE). Take the larger number, divide by efficiency for a gas heater's input rating, and have a pro confirm.
How many BTU to heat a pool?
8.34 BTU per gallon for every °F. Raising 15,000 gallons by 15 °F takes 1,876,500 BTU before any heat loss. Raising 10,000 gallons by 10 °F takes 834,000 BTU.
Sources
- U.S. DOE Energy Saver, Gas pool heaters (sizing formula, output range; archived copy): energy.gov via web.archive.org
- U.S. DOE Energy Saver, Heat pump swimming pool heaters (standard sizes, COP, air temperature; archived copy, June 2026): energy.gov via web.archive.org
- U.S. DOE Energy Saver, Swimming pool covers (archived copy, May 2026): energy.gov via web.archive.org