Home › Pool Heating Time Calculator
Pool Heating Time Calculator
Quick answer: Heating time equals gallons × 8.34 × degrees of rise divided by the heat that actually reaches the water each hour. For gas, that is the input rating times efficiency, minus what the surface loses while heating. A 20,000-gallon pool rising 17°F with a 250,000 BTU/h heater takes roughly a day.
How it works
The tool first finds the energy the water must absorb, then divides by net heat per hour. Net heat is the heater's input times its efficiency, reduced by a loss allowance for evaporation and wind during the heat-up. Covered and calm, the allowance can be near zero; open and breezy, 20 to 40 percent is a fair planning range. Heat pumps are rated by output, which falls as air cools, so enter the output at today's air temperature and 100 percent efficiency. For the background, read how long to heat a pool.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 20,000 gal, 65°F to 82°F, 250,000 BTU/h gas heater, 84% efficient, 20% loss allowance. Energy = 2,835,600 BTU. Net = 250,000 × 0.84 × 0.8 = 168,000 BTU/h. Time ≈ 16.9 h.
FAQ
Why is my real heat-up slower?
Cold nights, wind, a short pump schedule or a heater that needs service. See pool heater not heating.
Should the pump run the whole time?
The heater only works while water flows through it, so heat-up usually needs continuous circulation.
Does a cover help during heat-up?
Yes. Keeping it on while heating cuts evaporation, often the largest loss.
Back to the main calculator · Guides & FAQ