A trapezoid is a four-sided figure that has one pair of parallel lines and one pair of non-parallel lines. A 3-D trapezoid adds depth to this shape. Calculating the volume of a 3-D trapezoid is ideal for calculating the volume of a pool that slopes from the shallow end to the deep end. To calculate the volume you need to know the length, width, minimum height and maximum height of the trapezoid.

Calculate the area of the base of the trapezoid by multiplying the length times the width. For example, if you had a swimming pool that was 25 feet long and 20 feet wide, the base would be 500 square feet.

Add the minimum height of the trapezoid to the maximum height of the trapezoid. For example, if your pool had a minimum depth of 5 feet and a maximum depth of 9 feet, the average would be 7 feet.

Multiply the depth of the pool times the area from Step 1 to calculate the volume of the 3-D trapezoid. In this example, the area of the pool would be equal to 7 times 500, or 3,500 cubic feet.