Pool Heater Size Calculator in BTU/Hr & Energy Use

Heating water is pure physics: it takes 1 BTU to raise 1 pound of water 1°F, and water weighs 8.34 pounds per gallon. So the energy to warm your whole pool is gallons × 8.34 × temperature rise. The calculator works that out, divides by the hours you want the heat-up to take, and divides again by the heater's efficiency to give the input rating you should shop for.

Efficiency matters because no heater is perfect. A gas heater is about 82% efficient, so it must burn more fuel energy than ends up in the water. Heat pumps behave differently — they move heat rather than make it — but the sizing target here is the input BTU/hr a fired heater needs to hit your heat-up time. Faster heat-up means a bigger heater.

Real pools also lose heat to the air, especially at night and in wind, so treat this as the heat-up requirement and round up to the next standard heater size. The single biggest thing you can do is use a cover: it can cut heat loss by more than half, letting a smaller heater keep up. These are planning estimates — a dealer's heat-loss calculation is definitive.