The average 21-foot round pool with a depth of 48 inches holds 13,500 gallons of water. A 21-foot round pool with a depth of 52 inches holds 14,500 gallons, according to Robelle.

Measuring all dimensions in feet, the formula to calculate water capacity for round or oval pools is: average depth x length x width x 5.9. The formula for a square or rectangular pool is: average depth x length x width x 7.5. Accurately calculating water capacity allows owners to determine the correct amount and type of chemicals necessary to clean the pool and maintain the water's pH balance.