Estimate ideal pool heating time, water heat load, or required heater rating from water volume, current and target temperatures, and a constant delivered heat rate. Heat losses and fuel cost are not modeled.

Find the time needed to reach a higher water temperature.

Ideal estimate with no heat loss. Wind, evaporation and ambient conditions usually increase time. For heat pumps, use delivered thermal output; COP is not a percentage efficiency.

Input requires efficiency. Output already accounts for heater efficiency.

Pool Heat Time Formula

The following formula is used to calculate the total time it takes to heat a pool.

T = S × 8.34 × ΔT / P

Formula source: Pentair 2025–2026 Engineering Data, printed page 530. Use US gallons, a temperature rise in °F and delivered heat output in Btu/h. For an input rating, delivered output = input × efficiency / 100.

  • Where T is the heat time in hours
  • S is the water volume in US gallons
  • ΔT is the desired temperature rise (°F)
  • P is the constant delivered heat output (Btu/h), not a horsepower value

Multiply US gallons by approximately 8.34 Btu/(US gal·°F) and the temperature increase, then divide by delivered heat output. This bulk-water model ignores evaporation, wind, pool-structure heat capacity, solar gains and changing heater output; actual heat-up time can differ substantially.

Pool Heat Time

How long does it take to heat a pool?

The time it takes to heat a pool depends on three factors:

  1. The size of the pool. The larger the pool and larger the body of water, the more time it will take to heat the pool.
  2. The needed temperature rise. The more temperature difference you need to achieve, the longer it will take.
  3. The power output of the heater. A more powerful heater will reduce the time it takes to heat the pool.

Example Problem

How to calculate pool heat time?

  1. First, determine the size of the pool.

    For this example, the pool has 30,000 gallons of water.

  2. Next, determine the temperature rise that needs to be achieved.

    For this problem, we would like to raise the temperature 20 degrees F from where it is now.

  3. Next, determine the power output of the heater.

    The delivered thermal output is 400,000 Btu per hour. An input rating must first be multiplied by its efficiency fraction.

  4. Finally, calculate the time to heat the pool.

    Using the formula above, the pool heat time is calculated to be:
    T = S * 8.34 * ΔT / P
    T = 30,000 * 8.34 * 20 / 400,000
    T = 12.51 hours