The way you calculate the number of gallons in a pool depends on the type of pool you have. This is the general formula for calculating volume in gallons: length x width x depth x 7.5.
To calculate the volume of an irregularly shaped pool with a constant depth, it may be easier to split the pool into sections measuring each small area separately. For a pool with variable depths, you need to find the average depth. The formula then becomes length x width x average depth x 7.5 = volume (in gallons). For a circular pool, the formula is 3.14 x radius squared x average depth x 7.5 = volume (in gallons). Always remember to measure the height of the water level rather than the height of the pool wall.