How to use the pool heater calculator
- Enter your pool volume in gallons, litres or cubic metres.
- Enter the current and target water temperature.
- Choose the heater type and its size. Gas and propane heaters are sold by input BTU/h (for example 250,000 or 400,000); heat pumps by output.
- Enter the efficiency or COP and your fuel price to see the cost.
The formula
Heat needed (BTU) = Gallons × 8.34 lb × Temperature rise (°F) Heater output = Input BTU/h × Efficiency (gas, propane) Time (hours) = Heat needed ÷ Heater output Gas (therms) = Heat needed ÷ Efficiency ÷ 100,000 Propane (gallons) = Heat needed ÷ Efficiency ÷ 91,500 Heat pump (kWh) = Heat needed ÷ 3,412 ÷ COP
Worked example
A 20,000 gallon pool warmed from 70 °F to 80 °F:
- Heat = 20,000 × 8.34 × 10 ≈ 1.67 million BTU (489 kWh)
- 400,000 BTU gas heater at 84% → 336,000 BTU/h → about 5 hours
- Gas = 1,668,762 ÷ 0.84 ÷ 100,000 = 19.9 therms → about $30 at $1.50 per therm
- 125,000 BTU/h heat pump with COP 5 → 13.4 hours and 98 kWh → about $16 at $0.16 per kWh
Comparing heater types
| Type | Typical size | Efficiency | Best for |
|---|---|---|---|
| Natural gas | 150k–400k BTU/h input | 82–95% | Fast heat on demand, spas |
| Propane | 150k–400k BTU/h input | 82–95% | Homes without a gas line |
| Heat pump | 50k–140k BTU/h output | COP 3–6 | Lowest running cost in warm climates |
| Electric resistance | 5–30 kW | ~100% | Small spas and plunge pools |
Why real heating takes longer
The calculator gives the heat needed to warm the water once. While the heater runs, the pool keeps losing heat: evaporation from the surface is the largest loss, followed by radiation at night, wind and contact with the ground. On a cool, windy evening an uncovered pool can lose more heat than a heat pump adds. Expect real times to be 20–50% longer and plan to keep the heater running at a lower setting rather than heating from cold each weekend.
Ways to cut heating costs
- Use a solar cover whenever the pool is not in use — the biggest single saving.
- Lower the set point: each degree Fahrenheit higher costs roughly 10–30% more to maintain, depending on climate.
- Add a windbreak such as a fence or hedge.
- Run heat pumps during the day, when the air is warmest and the COP is highest.
Metric version of the formula
In metric units, heating water takes 4.186 kJ per kilogram per °C, which works out to about 1.16 kWh per cubic metre for every degree Celsius. A 50 m³ pool warmed by 4 °C therefore needs about 233 kWh of heat — about 26 m³ of natural gas in an 84% efficient heater (natural gas has roughly 10.5 kWh per m³) or, with a heat pump at COP 5, about 47 kWh of electricity.