The bottom of a pool always contains a liner to prevent the water from leaking. However, you cannot lay the liner down directly onto the ground. The liner is very smooth, so it needs a smooth surface to lie on. A pool liner laid down on uneven ground will experience a large degree of stress because of the water's weight bearing on the eccentricities between the liner and ground. Sand is an excellent medium to smooth out the ground because of its fine texture and affordable price. To use sand effectively, figure out how much you need.

Calculate how thick the sand will be. The layer of sand is usually 1 or 2 inches thick; it should be thick enough to work with and to smooth over any eccentricities.

Divide the pool (mentally) into its subcomponent shapes. Many pools are simply a rectangle or circle, but others have more complex geometries best described as a combination of rectangles, circles, and triangles.

Calculate the area of each shape. The area of a triangle is base multiplied by height. The area of a triangle is 1/2 its base multiplied by its height (where height is the length of the line running perpendicularly from its base to its vertex). The area of a circle is pi (approximately equal to 3.14) multiplied by the square of its radius.

Add the individual component areas together to obtain the total area of the pool.

Convert the desired height into a base unit that is in common with the calculated area. If you calculated the area in square feet, divide the number of inches by 12.

Multiply the desired height of the sand layer by the pool's area to get the total volume of required sand.
Tips & Warnings
 When ordering sand, you may have to express the quantity in a different unit of volume than what you used in your calculation, or even as a weight. A likely unit of volume is cubic yards; to convert cubic feet into cubic yards, divide the number of cubic feet by 27 (because 3 feet to a yard taken to the third power equals 27). If your supplier requires a weight, multiply the cubic yardage by 1.4 tons (assuming you are using Mason sand).
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References
 Photo Credit Swimming pool and pool house image by Jim Mills from Fotolia.com