🔥 Pool Heater Sizing Calculator
Estimate the heater output (BTU/hr) needed for your pool using a common contractor rule of thumb, based on surface area and desired temperature rise. Consult a pool professional for exact sizing, since wind, cover use, and climate also matter.
General estimate only — a pool professional can account for wind, cover use, and climate for precise sizing.
The 12 BTU rule of thumb
This page uses the contractor shortcut of 12 BTU per hour for every square foot of water surface, per degree Fahrenheit you want to gain:
BTU/hr = surface area (sq ft) × temperature rise (°F) × 12The defaults, 450 sq ft and a 20 °F rise, come to 108,000 BTU/hr. Depth is not asked for because almost all of a pool's heat leaves through the surface - evaporation, radiation and wind - so the surface is what the heater has to keep up with.
Reading the number you get
| Surface area | Rise | Heater output |
|---|---|---|
| 300 sq ft | 15 °F | 54,000 BTU/hr |
| 450 sq ft | 20 °F | 108,000 BTU/hr |
| 600 sq ft | 20 °F | 144,000 BTU/hr |
| 800 sq ft | 25 °F | 240,000 BTU/hr |
Heaters are sold in fixed steps - 150,000, 250,000, 400,000 BTU/hr and so on - so round up to the next model rather than down. An oversized heater reaches temperature sooner and then cycles off; an undersized one runs continuously and never quite gets there on a cold, windy week.
Frequently asked questions
What size heater do I need for a 16 x 32 ft pool?
That is 512 sq ft, so a 20 °F rise needs 512 × 20 × 12 = 122,880 BTU/hr. Round up and buy a 150,000 BTU/hr unit.
Why doesn't pool depth change the answer?
Depth changes how long the first heat-up takes, not the ongoing loss. A deep pool holds more water to warm, but once it is up to temperature it sheds heat through the same surface as a shallow one.
Is that figure the heater's input or its output?
Output - the heat actually delivered to the water. A gas heater at about 85 percent efficiency needs roughly 127,000 BTU/hr of input to put out 108,000, so check which rating a model is quoting.