Measure the length, width and depth of the area you want to cover in soil. For example, you may be building a flower bed that measures 12 feet by 3 feet by 18 inches deep.
Convert the measurements to yards by dividing feet by 3 and inches by 36. In this example, you would divide 12 feet by 3 to get 4 yards, 3 feet by 3 to get 1 yard and 18 inches by 36 inches per yard to get 0.5 yards.
Multiply the length times the width times the depth to calculate the number of cubic yards you will need to fill the desired area. In this example, you would multiply 4 by 1 by 0.5 to find that you would need 2 cubic yards of soil.