The correct way to calculate the gallons of water in a swimming pool is: Volume (in cubic feet) X  7.48 US Gallons per cubic foot = Total US Gallons Volume = Height (ft) X Width (ft) X Length (ft) This is roughly true for pools that are rectangular (including square) or free form, although some pool sites suggest using a multiplier of 7.5 instead. For Round pools and Oval pools, which are typical above ground shapes, you can get an approximate answer by finding: Length (ft) X Width (ft) X Depth (ft) X 5.9 = Total US Gallons