How to size a pool heater
BTU per hour = gallons × 8.34 × °F rise ÷ hours × 1.2
One BTU warms one pound of water by 1 °F, and a gallon of water weighs 8.34 lb. Raising 20,000 gallons by 20 °F takes 3.34 million BTU; spread over 24 hours that is 139,000 BTU per hour, and 166,800 with the 20 % allowance for heat lost from the surface while the pool warms up. Halve the time and the size doubles.
| Pool | +10 °F | +20 °F | +30 °F | 400k gas heater, +20 °F |
|---|---|---|---|---|
| 10,000 gal | 42,000 | 83,000 | 125,000 | 5 h |
| 15,000 gal | 63,000 | 125,000 | 188,000 | 7.4 h |
| 20,000 gal | 83,000 | 167,000 | 250,000 | 9.9 h |
| 25,000 gal | 104,000 | 209,000 | 313,000 | 12.4 h |
| 30,000 gal | 125,000 | 250,000 | 375,000 | 14.9 h |
| 40,000 gal | 167,000 | 334,000 | 500,000 | 19.9 h |
Keeping it warm: the surface-area method
Once the pool is warm, the heater only replaces what the surface loses. The US Department of Energy sizes for that: surface area in sq ft × (pool temperature − average air temperature of the coldest month you swim) × 12. A 16x32 pool (512 sq ft) kept at 82 °F in a 62 °F month needs 512 × 20 × 12 = 122,880 BTU per hour. The 12 assumes the heater can lift the water 1 to 1.25 °F an hour in a 3.5 mph wind; windy, dry sites and cold nights need more, and a solar cover reduces the loss.
Gas and propane heaters are sold from about 75,000 to 450,000 BTU per hour, so the larger sizes are the ones that heat a cold pool for a weekend. Home heat pumps mostly come in 75,000 to about 140,000 and are sized to hold a temperature, not to heat from cold in a day: see the pool heat pump calculator. Not sure of your gallons? The pool volume calculator gives them and the surface area; the pool pump size calculator checks the flow through the heater.
Formulas: 1 BTU per lb per °F, 8.34 lb per gallon; heat-up time assumes an 84 % efficient gas heater. Surface method, heater output ranges and the 1-1.25 °F per hour basis: US Department of Energy, Energy Saver pages on gas and heat pump pool heaters. Read October 23, 2026.
Frequently asked questions
What size heater for a 16x32 inground pool?
A 16x32 pool 3 to 8 ft deep holds about 21,000 gallons. Raising it 20 °F in 24 hours takes about 175,000 BTU per hour with a 20 % loss allowance. To keep it at 82 °F in a 62 °F month, the DOE method gives 512 × 20 × 12 = 122,880 BTU per hour.
How long does it take to heat a pool from 60 to 80?
A 20,000-gallon pool needs 3.34 million BTU for a 20 °F rise. A 400,000 BTU gas heater at 84 % efficiency delivers that in about 10 hours, plus surface losses; a 250,000 BTU heater takes about 16 hours. A heat pump of 140,000 BTU needs 24 hours or more, longer in cool air.
How do you calculate the BTU needed to heat a pool?
Multiply gallons by 8.34, the pounds per gallon, and by the degrees of rise: that is the total BTU. Divide by the hours you allow to get BTU per hour, then add about 20 % for heat lost from the surface while the pool warms up.
How do I size a heater to keep my pool warm?
Use the US Department of Energy method: surface area in sq ft × (pool temperature − average air temperature of the coldest month you swim) × 12. The 12 assumes a 1 to 1.25 °F rise per hour in a 3.5 mph wind. Windy sites, low humidity and cool nights need a larger heater.
Is a bigger pool heater better?
For gas heaters, a larger size mostly means faster heat-ups: the fuel burned per degree is about the same, so it does not cost much more to run. The limits are the purchase price, the gas line and meter capacity, and the flow your pump sends through the heater.