Pool Heater Sizing Calculator

8.33 pounds per gallon is the weight of water at typical pool temperatures, and it's the standard conversion factor used throughout water-heating engineering — combined with the definition of a BTU (the energy to raise one pound of water by one degree Fahrenheit), multiplying gallons by 8.33 by the desired temperature rise gives the total heat energy required, in BTUs, for that warm-up. This is the same basic formula used for sizing water heaters generally, not something specific to pools.

A pool's water surface constantly loses heat through evaporation, and wind accelerates that process by continuously sweeping away the warm, moisture-saturated air layer that forms just above calm water, replacing it with cooler, drier air that draws off heat faster. An exposed, windy pool location can lose noticeably more heat than a sheltered one at the same air temperature, which is why this calculator applies a higher heat-load multiplier for exposed sites even when temperature inputs are identical.

Heaters are manufactured in standard size increments, so the calculator has to round up to an actual available product size regardless of the exact calculated load. The 10% safety margin on top of that additional exists because real-world conditions (a colder-than-typical night, extra wind, an uncovered pool losing more heat than assumed) can push the actual heat load above the calculated baseline, and an undersized heater will struggle to maintain temperature during those periods, so sizing with headroom is standard practice.

The cost estimate assumes a representative national-average natural gas price and a typical 4 hours/day of heater run time, but your actual gas rate varies by region and utility, and actual run time depends heavily on factors this calculator doesn't model directly — a pool cover dramatically cuts evaporative heat loss and can reduce run time substantially, while an uncovered pool in a cold, windy climate will run longer than 4 hours/day to hold temperature.

They drive two different parts of the sizing calculation. Current Water Temp only matters for the one-time initial warm-up — how far the water itself has to rise to reach your desired temperature — and stops mattering once the pool is at temperature. Average Ambient Air Temp matters continuously, because it sets how much heat the pool's surface keeps losing to the surrounding air every hour the pool is held at its target temperature, which is what actually determines the recommended heater size, not the one-time warm-up energy.