Pool Heater Size Calculator

The physics of heating a pool

By definition, one British thermal unit (BTU) warms one pound of water by 1 °F, and a US gallon of water weighs about 8.34 lb. So the heat a pool needs is simply its weight of water times the temperature rise. In metric, it takes 1.163 watt-hours to warm one litre by 1 °C. Divide that heat by the heater's output per hour and you get the heat-up time.

Heat needed (BTU) = US gallons × 8.34 × temperature rise (°F)
Heat needed (kWh) = litres × temperature rise (°C) × 0.001163
Hours = heat needed ÷ heater output per hour
Gas heater output = rated input × efficiency · 1 kW = 3,412 BTU/hr

Worked examples

• 15,000 gallons from 65 °F to 82 °F needs 15,000 × 8.34 × 17 ≈ 2.13 million BTU (624 kWh). A 400,000 BTU/hr gas heater at 84% efficiency puts 336,000 BTU/hr into the water: about 6.3 hours. A heat pump rated at 110,000 BTU/hr output takes about 19.3 hours. To do it in 24 hours you need at least 88,700 BTU/hr of output.
• 50,000 litres from 18 °C to 28 °C needs 50,000 × 10 × 0.001163 = 581.5 kWh. A 13 kW heat pump takes about 45 hours, which is why heat-pump pools are usually warmed up over two or three days and then kept warm.
• A 400-gallon hot tub from 60 °F to 102 °F needs 140,112 BTU (41 kWh). A 4 kW heater (13,650 BTU/hr) takes about 10.3 hours.

Why real heating takes longer

These times assume all the heat stays in the water. In reality an outdoor pool loses heat the whole time, mostly through evaporation from the surface, which the US Department of Energy identifies as by far the largest source of heat loss in pools. Wind, low humidity and cool nights increase it. Practical consequences:

• Treat the result as a minimum. On a windy, cool day the real time can be much longer.
• A pool cover is the single most effective way to cut heat loss and heating cost; DOE notes that covers can reduce pool heating costs substantially.
• Heat pumps lose capacity as the air gets colder, so their rated output (often measured at mild air temperatures) may not be reached in spring or autumn. Check the manufacturer's output table for your conditions.

Choosing a heater size

Heater makers and pool professionals size heaters from the pool's surface area, your climate, wind exposure and how quickly you want to heat up, because surface heat loss, not the volume, decides how much power is needed to keep the pool warm. Use the time-based result here to check whether a heater can warm the pool fast enough for the way you use it (for example, weekend-only heating needs a larger heater than keeping the pool warm all season), and ask a professional for a full sizing analysis before you buy.

• Gas heaters (natural gas or propane) heat fast in any weather and suit occasional heating.
• Heat pumps cost much less per unit of heat (see the pool heating cost calculator) but heat slowly and work best in mild climates for season-long heating.
• Electric resistance heaters are common in hot tubs, where the water volume is small.

Common mistakes

• Using input instead of output. A gas heater's rating is its fuel input; multiply by efficiency. A heat pump's rating is usually already its heat output.
• Mixing up kW and BTU/hr. 1 kW = 3,412 BTU/hr.
• Underestimating volume. Check it with the pool volume calculator.

Estimates ignore heat loss and are a minimum. Ask a pool professional for a full sizing analysis.

Frequently asked questions

Sources

• US Department of Energy, Energy Saver: gas pool heaters, heat pump pool heaters and swimming pool covers (sizing by a professional, efficiency, covers as the most effective way to cut heating costs).
• US DOE fact sheet (archived): Heat pump swimming pool heaters (evaporation as the largest source of pool energy loss).
• NIST: 1 BTU (IT) = 1,055.06 J; 1 kW = 3,412.14 BTU/hr; specific heat of water ≈ 4.186 kJ/(kg·K).

Last reviewed 28 October 2026. Results are estimates; see our disclaimer.