Pool Heat Pump Sizing Calculator
Size a pool heater from the pool's surface area and your desired temperature rise, using a standard BTU-per-square-foot rule of thumb.
How this calculator works
Sizing a pool heater starts with how much surface area the pool has to lose heat from and how many degrees you want to raise the water temperature. This calculator uses a standard rule-of-thumb multiplier of 12 BTU per square foot per degree of desired rise to estimate the heater output needed.
Formula: Heater size (BTU/hr) = pool surface area (sq ft) × desired temperature rise (°F) × 12.
Worked example
- Pool surface area: 450 sq ft
- Desired temperature rise: 20°F
- Heater size: 450 × 20 × 12 = 108,000 BTU/hr
Notes
Wind, an uncovered pool, and a cooler climate all pull heat out of the water faster than this base rule of thumb accounts for, which effectively calls for a bigger heater than the calculated figure to hit and hold your target temperature.
A pool cover cuts heat loss substantially by blocking evaporation, which is the single biggest source of heat loss on most pools, so using one regularly can let you get away with a smaller, less expensive heater than an uncovered pool would need.
How to use
Enter your pool’s surface area in square feet and the number of degrees you want to raise the water temperature above its baseline. The calculator multiplies these by the standard 12 BTU/sq ft/°F factor to give you a starting BTU/hr output for shopping heat pumps or gas heaters.
A rule-of-thumb planning estimate. Wind exposure, pool covers, and climate affect actual heater sizing — confirm with a pool equipment dealer.
Frequently asked questions
What does BTU/hr mean for a pool heater?
BTU stands for British Thermal Unit, a measure of heat energy, and BTU/hr describes how much heat a unit can add to the pool every hour it runs. A higher BTU/hr rating means the heater can raise the pool's temperature faster or handle a bigger pool for the same temperature rise.
Does this calculator work for both gas heaters and heat pumps?
The BTU/hr output figure applies to sizing either type, since both are rated in BTU/hr. Keep in mind that heat pumps are more sensitive to outdoor air temperature than gas heaters, so a heat pump may need to be sized larger, or paired with a cover, to perform well in cooler weather.
Why does pool surface area matter more than pool volume for heater sizing?
Heat loss happens mainly at the water's surface, largely through evaporation, not throughout the pool's full volume. A shallow pool with a large surface area loses heat faster and needs more heating capacity than a deep pool with the same surface area, which is why this calculation is based on surface area rather than total gallons.
How much does a pool cover actually help with heater sizing?
A pool cover blocks evaporation, which is typically the largest source of heat loss in an outdoor pool, so using one consistently reduces how much heat the water loses between heating cycles. This can let a smaller, less expensive heater maintain your target temperature compared with running the same pool uncovered.
Estimates only. Verify quantities with your supplier before purchasing.