Pool heater size calculator
Heater size depends on surface area and how far above the air temperature you want the water. This uses the US Department of Energy sizing formula, then estimates heat-up time.
Heater output needed
DOE’s formula assumes a 3.5 mph average wind at the surface. Windy, dry or cool-night sites need a bigger heater; a cover lets you get away with a smaller one.
How long to heat the water
About – hours for 15,000 gallons, before surface losses (use a cover while heating).
The math
Sizing: Btu/h = surface area (ft²) × temperature rise (°F) × 12. This is the approximate formula published by the US Department of Energy for outdoor pools, based on a 1–1.25 °F per hour heat-up and a 3.5 mph wind; multiply by 1.5 or 2 for faster heat-up.
Heat-up: one gallon of water needs 8.34 Btu to warm by 1 °F. Hours = gallons × 8.34 × rise ÷ (input × efficiency).
Gas heater or heat pump?
| Heat pump | Gas | |
|---|---|---|
| Running cost | Low | High |
| Heat-up speed | Slow | Fast |
| Cold weather | Weak below ~50 °F air | Works in any weather |
| Best for | Season-long heating | Spas, occasional use |
Questions pool owners ask
What size heater do I need for my pool?
The US Department of Energy’s rule of thumb is pool surface area (ft²) × temperature rise (°F) × 12 = Btu/hour of heater output. The temperature rise is your desired pool temperature minus the average air temperature of the coldest month you swim. A 16 × 32 ft pool (512 ft²) heated 20 °F above that needs about 123,000 Btu/h.
How long does it take to heat a pool?
Heating a volume of water takes about 8.34 Btu per gallon per °F. A 20,000 gallon pool raised 10 °F needs 1.67 million Btu; a 250,000 Btu gas heater at 82% efficiency (about 205,000 Btu/h output) needs roughly 8 hours, plus extra to cover heat lost from the surface meanwhile. A cover cuts that loss dramatically.
Is a heat pump or gas heater better?
Heat pumps cost much less to run in warm and mild weather because they move heat from the air instead of burning fuel, but they heat slowly and lose output when air drops below roughly 50 °F. Gas heaters heat fast in any weather, which suits spas and occasional use, but cost more per degree.
Do I need a pool cover with a heater?
It’s the single best upgrade. Most heat loss from a pool is evaporation from the surface. The US Department of Energy recommends a cover as the most effective way to reduce pool heating costs.