The evaporation rate depends on the surface area of the pool, the temperature, and the relative humidity and the wind. "Splash-out" water is probably a much more significant source of water loss than evaporation. To calculate evaporation, mark the water level on the wall of the pool. After some time, mark the water level again. It should be lower than the original one. Measure the area of the pool and then multiply it by the difference of the two water levels. The result is the amount of water evaporated.
It is important to remember to turn off the pool's auto fill for this process. You must make all measurements using the same units. If the pool is 10 feet by 20 feet then you would multiply 120 inches by 240 inches to get 28,800 square inches. Multiply this by the inches of water lost: for example, 28,800 x 0.5 inch lost to get 14,400 cubic inches of total water lost. Multiply this by 0.00433 to convert to U.S. Gallons. 14,400 cu. in. of water equals 62.352 Gallons (US liquid).