How many gallons are in a swimming pool?

Length x Width x Average Depth = volume for a rectangular pool

Average depth x 0.5 diameter x 0.5 diameter x 3.1416 = volume of circular pool

A swimming pool that measures 12 feet by 30 feet and has an average depth of 6 feet contains approximately 16,157 gallons of water.

12 x 30 x 6 = 2,160 cubic feet

1 cubic foot = approximately 7.48 gallons

2160 x 7.48 = 16,156.8 gallons

A circular pool that has a diameter that measures 20 feet and an average depth of 6 feet contains approximately 14,100 gallons of water.

6 x 0.5 x 20 x 0.5 x 20 x 3.1416 = 1885 cubic feet

1885 x 7.48 = 14,099.5 gallons

See related link for unit of measure conversion tool.