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Pool Heater Size Calculator
How warm, how fast: three answers give the BTU your heater must deliver.
1 of 3: your gallons
The heater output appears here.
- 8.34 BTU
- warms a gallon of water by 1°F
- Output
- compare the BTU a heater delivers, not its input
- Cover
- keeps the heat you paid for
Enter your pool's gallons, how many degrees warmer you want it, and how long you're willing to wait, and the calculator will suggest a pool heater size. BTU measures heat, and it takes 8.34 BTU to warm a single gallon of water one degree F. When you compare heaters, go by the BTU they actually put into the water, not the BTU they consume.
- Heat needed: 8.34 BTU to warm 1 gallon by 1 degree F
- Heat pump: each unit of power yields 5 units of heat
- Gas heater: of every 100 BTU of gas burned, about 82 warm the water
How big a heater does my pool need?
The answer hinges on 3 things: the gallons in your pool, the rise in temperature you want, and how long you'll wait for it. More gallons, more degrees, or less patience each push the BTU per hour higher.
Take a 20,000-gallon pool. Warming it by 10 degrees F across 24 hours takes a heater putting out around 70,000 BTU per hour.
Finding your pool's gallons
With a rectangular pool, multiply length by width, both in feet, for the surface area. Multiply that by the average depth, then by 7.48. For round and oval pools, swap 7.48 for 5.9.
When the bottom slopes, split the difference between your shallow end and your deep end to get the average. One US gallon works out to 3.785 liters.
At an average depth of 4.5 ft, a 12 x 24 ft pool holds about 9,690 gallons, a 16 x 32 ft pool about 17,200, and a 20 x 40 ft pool about 26,900.
Heater BTU for each pool size
These numbers assume you want the heating done in 24 hours and show the heat the heater must put into the water each hour.
| Pool size (gallons) | BTU per hour to warm 10°F | BTU per hour to warm 20°F |
|---|---|---|
| 10,000 | 35,000 | 70,000 |
| 20,000 | 70,000 | 139,000 |
| 25,000 | 87,000 | 174,000 |
| 40,000 | 139,000 | 278,000 |
Want the pool warm in 12 hours rather than 24? Then double the output in the table.
Pool heat pump sizing
A heat pump uses electricity to pull warmth from the air and move it to the water. For every 1 unit of power it uses, it delivers about 5 units of heat, so a heat pump putting out 70,000 BTU an hour uses only about a fifth of that in electricity.
Check the heat output on the label, not the power draw, and match it to your pool's number in the table.
Gas heater output and input
A gas heater runs at about 82% efficiency, so roughly 82 of every 100 BTU of gas burned end up in the water.
A gas heater's label usually shows the BTU it burns, so take 82% of that to see what reaches the water and compare it with your pool's number in the table. If your pool needs 70,000 BTU an hour, look for a heater whose burn rating, times 82%, comes to at least that.
How long does a pool take to heat up?
To estimate heating time, divide the total heat needed by the heater's hourly output. For a 20,000-gallon pool, a 10°F rise calls for 20,000 times 8.34 times 10 BTU, so at 70,000 BTU per hour it takes about 24 hours, not counting heat the pool loses while it warms.
For a different pool or temperature rise, enter your numbers in the calculator.
How this is worked out
Multiply your pool's gallons by 8.34 and by the degrees you want to add, and you get the total heat needed. Divide that total by the hours you'll wait to find the output your heater must deliver each hour.
- Heat required overall = gallons times 8.34 times the degrees F
- Hourly output = the overall heat divided by the hours
- Gas heater input = the output divided by 82%
- For a rectangular pool, gallons = length times width times average depth times 7.48; for a round or oval pool, use 5.9 instead of 7.48