When pouring concrete it is necessary to know how much concrete you need in order to purchase the appropriate amount. Concrete is sold by cubic yards, and one cubic yard of concrete will usually fill a slab measuring 36 inches in length, width and height. You can calculate the amount of concrete you need by employing a mathematical formula.
Things You'll Need
 Measuring tape
 Calculator

Know that the formula for calculating concrete consists of the following: ((length x width x height)/27) x 1.05 = Total concrete needed plus 5 percent extra. The length, width and height are measured in feet. Generally, you should purchase 5 percent more concrete than you need to complete your concrete project to ensure that you have enough to amend mistakes.

Measure the length, width and height of the area to be filled. For instance, in this example, let's assume that the length is 100 feet, the width is 100 feet and the height is 1/3 foot.

Multiply the length by the width by the height in the following manner to determine how many cubic feet of concrete you need: 100 x 100 x 1/3 = 3333.33 cubic feet.

Divide the above product by 27 (3 cubic feet x 3 cubic feet x 3 cubic feet) in the following manner to determine the number of cubic yards of concrete you need: 3333.33 cubic ft/27 cubic ft = 123.456 cubic yards.

Multiply the cubic yards needed by 1.05 in the following manner to derive the total concrete needed plus 5 percent extra: 123.456 X 1.05 = 129.629 cubic yards.
Tips & Warnings
 If you measure in inches, convert the inches to feet before completing the formula.
 Don't forget to convert cubic feet to cubic yards otherwise you will not purchase the correct amount of concrete needed for your project.