Pool Evaporation Calculator: Daily & Weekly Water Loss

Water leaves a pool mostly through its surface, so evaporation loss is surface area times an evaporation rate. The rate is a depth of water lost per day — a typical uncovered pool loses roughly a quarter inch (6 mm) a day, though heat, wind, low humidity, and a running heater can push it much higher. The calculator multiplies your surface area by the rate and converts to gallons.

The level drop is the same everywhere on the surface — it equals the rate directly — but the volume lost scales with area, so a big pool loses many more gallons for the same quarter-inch drop. A 32 × 16 ft pool at a quarter inch a day loses about 80 gallons daily, close to 560 gallons a week.

This is the number to check before you panic about a leak. Big overnight drops in hot, dry, or windy weather are usually evaporation, not a crack. If losses far exceed this estimate, confirm with a bucket test — float a bucket of pool water on the steps and compare the two drops. A cover is the fix either way: it can cut evaporation by up to 90%.