Quick Scoop: The size pool heater you need depends mainly on your pool’s gallons, your climate, and how fast you want the water to warm up. A common shortcut is that bigger pools and cooler weather require higher BTU output, with many residential gas heaters landing in the 80,000 to 400,000 BTU range.

What To Use

For a fast estimate, pool heater sizing guides commonly recommend matching heater size to pool volume and desired heating speed. One reference shows approximate gas heater ranges like 10,000 gallons: 40,000–60,000 BTU , 15,000 gallons: 60,000–90,000 BTU , 20,000 gallons: 80,000–120,000 BTU , and 25,000 gallons: 100,000–150,000 BTU.

Quick Rule

A useful rule of thumb from one guide is to multiply pool gallons by about 4 BTU per gallon to estimate the minimum needed for a typical target temperature on a mild day. That means a 15,000-gallon pool would need roughly 60,000 BTU as a baseline, while colder weather or faster heating usually calls for more.

Size Examples

A published chart gives these common pairings for rectangular pools: 16' x 32' = 250,000 BTU , 18' x 36' = 325,000 BTU , and 20' x 40' = 400,000 BTU. It also notes that smaller heaters can be underpowered for larger pools, especially if you want quicker warm-up times.

What Changes The Answer

  • Pool volume: more gallons need more BTUs.
  • Climate: colder air and wind increase heat loss.
  • Heating goal: raising temp a few degrees is easier than extending swim season in cool weather.
  • Heater type: gas heaters and heat pumps are sized differently, so the “right” size depends on the equipment.

Best Next Step

If you want, send me your pool length, width, average depth, and your location/climate , and I can estimate the heater size more precisely from those numbers.