About the Pool Heater Size Calculator
This pool heater size calculator estimates how many BTU per hour a pool heater must deliver for your pool. It checks two separate jobs and sizes the heater for whichever is bigger: raising the whole volume of water from its starting temperature to your target within the time you choose, and keeping up with the heat the surface loses to cooler air during the swimming season.
Use it when buying a new gas heater, choosing between heat pump models, or checking whether an existing heater is big enough. Enter the pool’s length, width and average depth, the water temperature you start from, the average air temperature of the coldest month you want to swim in, and how quickly you want the pool warm. The tool then converts the required output into a gas input rating (using burner efficiency) or an electrical draw (using a heat pump’s COP) and suggests the next common size up.
The surface-loss check uses the long-standing industry rule of thumb of 12 BTU per hour per square foot per °F of difference between water and air, which assumes an uncovered pool and light wind. Windy, shaded or high-altitude sites lose more heat; a solar cover cuts losses sharply, so a covered pool can often use a smaller heater. Treat the result as a starting point to discuss with your pool professional.
With the default inputs, the required heater output is 122,880 BTU/h. Change any value above to recalculate instantly.
How to use the pool heater size calculator
- 1Enter the pool’s length, width and average depth in feet.
- 2Enter the current water temperature and the temperature you want.
- 3Enter the average air temperature of the coldest month you plan to swim.
- 4Choose how many hours you are willing to wait for the initial heat-up.
- 5Pick the heater type and efficiency or COP to see the input rating or electrical draw and a suggested size.
Formula and method
Pool volume V in gallons is length × width × average depth × 7.48. One gallon of water weighs 8.34 lb and needs 1 BTU per pound to rise 1 °F, so the heat-up output is the total heat divided by the hours you allow. The surface rule of thumb, 12 BTU/h per square foot per °F between water and coldest-month air, approximates evaporation, convection and radiation losses from an uncovered pool in light wind.
The heater must cover the larger of the two figures. A gas heater’s nameplate input rating equals that output divided by its efficiency; a heat pump’s electrical draw equals the output divided by its COP and by 3,412 BTU per kWh; an electric resistance heater converts 1 kW into 3,412 BTU/h. The heat-up figure ignores losses during warm-up, so allow a margin if you need the pool warm on a deadline.
- V
- Pool volume in US gallons
- A
- Pool surface area in square feet
- t
- Heat-up time in hours
- T
- Temperatures in °F (start water, target water, coldest-month air)
Worked examples
16 × 32 ft pool heated with gas
The pool holds 512 × 5 × 7.48 ≈ 19,150 gallons. Heating it 17 °F in 24 hours needs 19,150 × 8.34 × 17 ÷ 24 ≈ 113,129 BTU/h, but holding 82 °F against 62 °F air needs 512 × 20 × 12 = 122,880 BTU/h, so surface loss sets the size. At 84% efficiency that is about 146,286 BTU/h input, so a 150k heater fits.
Large pool, fast warm-up with a condensing heater
A 36 × 18 ft pool with a 4.5 ft average depth holds about 21,813 gallons. Raising it 24 °F in a day needs about 181,922 BTU/h, more than the 132,192 BTU/h surface loss. A 95%-efficient heater must burn about 191,497 BTU/h, so the 200k size is the match.
Heat pump with a three-day warm-up
Heat pumps are usually given a few days to warm a pool, so the heat-up need is small (about 21,836 BTU/h). Surface loss against 68 °F air is 450 × 16 × 12 = 86,400 BTU/h. At a COP of 5 the heat pump draws about 5.06 kW, and the next common size up is a 100k BTU/h unit.
Frequently asked questions
What size pool heater do I need?+
Most residential in-ground pools use gas heaters of 150,000–400,000 BTU/h input or heat pumps of 75,000–140,000 BTU/h output. The right size depends on surface area, how far the water must be raised above the air temperature and how quickly you want it warm, which is what this calculator works out.
Is it bad to oversize a pool heater?+
A gas heater that is larger than needed heats the pool faster and costs little more to run per BTU, so modest oversizing is common. The trade-off is a higher purchase price and, for heat pumps, a larger electrical circuit. Undersizing is the bigger problem because the heater may never reach your target on cool days.
How long does it take to heat a pool?+
Divide the heat needed (gallons × 8.34 × degrees of rise) by the heater’s output. A 20,000-gallon pool raised 20 °F needs about 3.3 million BTU, so a 150,000 BTU/h output takes roughly 22 hours before losses, while a 100,000 BTU/h heat pump takes about 33 hours or longer in cool weather.
Does a pool cover change the heater size?+
Yes. Most heat loss from a pool is evaporation at the surface, and a solar or insulating cover blocks much of it overnight. With a cover used consistently, the surface-loss requirement drops and a smaller heater or heat pump can hold temperature, though you still need enough output for the initial warm-up.
What is the difference between input and output BTU?+
Gas heaters are labelled by input BTU, the energy in the gas they burn. Output is input multiplied by efficiency, so a 200,000 BTU/h heater at 84% delivers about 168,000 BTU/h to the water. Heat pumps are labelled by output because they move heat rather than burn fuel.