How long to heat the pool.

Worked example

A 10,000 gallon pool from 70 °F to 80 °F: 10,000 × 8.34 × 10 = 834,000 BTU (about 244 kWh of heat). A 100,000 BTU/h gas heater at 80 percent efficiency delivers 80,000 BTU/h, so the ideal time is 834,000 / 80,000 = 10.4 hours. With a 20 percent heat loss allowance, plan on about 12.5 hours.

How it works

A US gallon of water weighs about 8.34 lb and one BTU raises one pound of water by one °F. BTU needed = gallons × 8.34 × temperature rise (°F). A rise in °C is multiplied by 1.8 first. Hours = BTU needed / (heater BTU per hour × efficiency). The loss allowance multiplies that time by (1 + loss%). One kWh is 3,412 BTU.

What this is not. Real heating is slower than the ideal figure. Evaporation, wind, cool nights, a cold deck and no cover all pull heat out while the heater works, and the water mass of a cold pool takes time. A solar cover can cut heat loss a lot. Heat pumps lose output as the air gets cooler. Gas heaters need proper venting and installation by a qualified person; follow the manufacturer’s instructions.

Sources and basis (checked 2026-10-06): the specific heat of water, 1 BTU per pound per °F, and 8.34 lb per US gallon, are standard physical values; 1 kWh = 3,412.14 BTU. Heater output is the rated figure you enter, so real heating time also depends on heat loss, which this does not model.

FAQ

How many BTU does it take to heat a pool?

Gallons times 8.34 times the temperature rise in degrees Fahrenheit. Heating 10,000 gallons by 10 degrees takes 834,000 BTU.

How long does it take to heat a pool 10 degrees?

It depends on pool size and heater. A 10,000 gallon pool needs about 10 hours at 80,000 BTU per hour of delivered heat, and more once heat loss is included. Larger heaters take proportionally less.

Why is my pool slower than the calculator?

Heat loss. Evaporation, wind and cool nights remove heat as fast as you add it. Cover the pool to cut that loss.

Does a heat pump work differently?

The arithmetic is the same: enter the heat output rating in BTU per hour and 100 percent efficiency. Output drops as outside air cools, so use the rating at your actual air temperature.