Use this formula: Volume =length x depth x width (for the depth, take the average of the deep end and the shallow end. If your pool is 8 feet deep at one end and 3 feet deep at the other, use 5.5 feet for the depth).
Example: A 36' x 18' x 5.5' pool: 36' x 18' x 5.5' = 3,564 cubic feet. Now to go a bit further, you might want to know what that means in terms of gallons of water. If so, use this information:
One cubic foot equates to about 7.48 gallons. Therefore, using the pool example above you get this: 3,564 cubic feet x 7.48 gallons = about 26,700 gallons of water.