Enter your pool volume, the temperature rise you want and how fast you want it, to get the heater output in BTU per hour.
The formula
Water needs 8.34 BTU to raise one gallon by 1°F. Total heat = gallons × 8.34 × temperature rise; divide by the hours you allow, then multiply by 1.2 to cover evaporation and surface losses along the way. That 20% allowance suits an uncovered pool in mild wind — a solar cover shrinks it dramatically.
Worked example
A 15,000-gallon pool raised 15°F in 24 hours: 15,000 × 8.34 × 15 = 1,876,500 BTU ÷ 24 × 1.2 ≈ 93,800 BTU/hr — squarely in the range of a common 100k BTU gas heater.
Using the number
Pick the next heater size up from the answer; undersized heaters run constantly and still lose ground on windy nights. Gas heaters deliver their rated BTU regardless of weather, while heat pumps are cheaper to run but slow down as air temperature falls — the same math applies, just over more hours. A cover is the cheapest upgrade of all: it can cut overnight losses by half or more.