Concrete is sold in cubic yards. One cubic yard of concrete would fill a box 36 inches long by 36 inches wide by 36 inches high. To calculate the right amount of concrete you need for a project, your need to determine the dimensions of the final slab. You need to calculate all your measurements in the same unit and follow a specific mathematical formula to determine the right amount of concrete needed.

Use a steel tape measure to measure the area of the project. Measure the width and the length of the space. The depth of the slab for your project depends on the use of the slab. For a driveway, for example, you would need 4 to 5 inches depth. For a sidewalk, 3 inches will suffice.

Convert the measurements for length and width into inches.

Multiply the length by the width by the depth. For example, if your area for the slab is 10 feet 6 inches in width by 27 feet 4 1/2 inches in length by 4 inches in depth, then:

10 feet 6 inches = 126 inches 27 feet 4 1/2 inches = 328.5 inches 4 inches = 4 inches

126 x 328.5 x 4 = 165,564 cubic inches

Convert the cubic inches into cubic yards. A cubic yard in inches equals 46,656. (36 x 36 x 36= 46,656) Divide your amount of cubic inches by one cubic yard or 46,656.

165,564 divided by 46656 = 3.55 cubic yards

Purchase slightly more than your calculations indicate. Because of the organic nature of concrete (a mixture of gravel, cement and water), a physical cubic yard of concrete is a close approximation and not an exact measure.