Pool Heater BTU Calculator
How Many BTUs Does a Pool Heater Need?
Pool heater sizing is measured in BTU per hour. The higher the BTU output, the faster the heater warms your water. The two factors that matter most are your pool surface area and the temperature difference between your desired water temperature and the coldest air temperature during your swim season.
Pool Heater BTU Formulas
Method 1: Surface area method
BTU/hr = Pool surface area (sq ft) x Temperature rise (°F) x 12
Example: 512 sq ft x 30°F x 12 = 184,320 BTU/hr.
Method 2: Volume method
BTU/hr = Gallons x 8.34 x Temperature rise / Heat-up hours
The calculator uses both methods and takes the higher result, which is the industry-recommended approach.
Pool Heater BTU by Pool Size
| Pool Size | Surface Area | Temp Rise 20°F | Temp Rise 30°F | Temp Rise 40°F |
|---|---|---|---|---|
| 12 x 24 ft | 288 sq ft | ~69K BTU | ~104K BTU | ~138K BTU |
| 16 x 32 ft | 512 sq ft | ~123K BTU | ~184K BTU | ~246K BTU |
| 18 x 36 ft | 648 sq ft | ~156K BTU | ~233K BTU | ~311K BTU |
| 20 x 40 ft | 800 sq ft | ~192K BTU | ~288K BTU | ~384K BTU |
| 24 ft round | 452 sq ft | ~109K BTU | ~163K BTU | ~217K BTU |
Types of Pool Heaters
Gas pool heaters
Gas pool heaters heat pools quickly regardless of outside air temperature. Range: 100K to 400K+ BTU/hr. Best for: cold climates, occasional use, fast heat-up.
Pool heat pumps
Heat pumps are 3 to 6 times more energy efficient than gas. Best when air temps are above 50°F. Best for: year-round warm climates, frequent use.
Solar pool heaters
Zero running cost but dependent on sunlight. Require collectors covering 50 to 100% of pool surface area. Usually paired with gas or heat pump backup.
Electric resistance heaters
Electric heaters suit small pools and spas only. Running costs are 3 to 5 times higher than gas per BTU.
Pool Heater Running Costs
| Heater type | 100K BTU/hr | 200K BTU/hr | 400K BTU/hr |
|---|---|---|---|
| Natural gas | ~$1.50/hr | ~$3.00/hr | ~$6.00/hr |
| Propane | ~$2.75/hr | ~$5.50/hr | ~$11.00/hr |
| Heat pump (COP 5) | ~$0.59/hr | ~$1.18/hr | ~$2.36/hr |
| Electric resistance | ~$4.84/hr | ~$9.68/hr | ~$19.36/hr |
Based on 2025 US average prices. A solar pool cover reduces running costs by 50 to 70%.
Top Pool Heater Brands
Hayward, Pentair, and Raypak are the top three brands. For heat pumps, Hayward HeatPro and Pentair UltraTemp are industry leaders.
Frequently Asked Questions
What size pool heater for a 15,000-gallon pool?
For a 15,000-gallon pool with a 30°F rise, you need about 135,000 BTU/hr minimum. Most owners choose a 150,000 or 200,000 BTU/hr heater.
What size pool heater for a 20,000-gallon pool?
A 20,000-gallon pool needs approximately 162,000 to 184,000 BTU/hr with a 30°F rise. Choose 200,000 BTU/hr, or 250,000 BTU/hr for cold climates.
How long does it take to heat a pool?
A 150,000 BTU gas heater raises a 15,000-gallon pool by about 1°F per hour. A 20°F rise takes roughly 20 hours. Use the calculator above for your exact estimate.
Gas heater vs heat pump: which is better?
Gas heaters heat faster in any temperature but cost 4 to 8 times more per hour. Heat pumps cost far less to run but lose efficiency below 50°F. For frequent year-round use in a warm climate, a heat pump saves money over time.
How much does it cost to run a pool heater per month?
A 200,000 BTU natural gas heater running 8 hours per day costs about $72 to $90 per month. A heat pump costs about $28 to $57 per month. A pool cover reduces these costs by 50 to 70%.