Here's the formula for calculating volume of water in your pool (with varying depth):

If the pool floor slopes uniformly from end to end, the average depth can be found by adding the deepest and shallowest depth measurements and dividing the results by two. If the pool's dimensions are in feet, the volume will be in cubic feet. One cubic foot of water equals 7.5 gallons.
Example: The water volume of a pool 60 ft. long, 30 ft. wide and that slopes in depth from 3 ft. to 10 ft. is as follows:
30 x 60 x ((10 + 3)/2) = 11,700 cubic ft. of water11,700 x 7.5 = 87,750 gallons