Define your project as a shape or a combination of shapes, and add these together. You will need to find the total volume of the shapes by measuring the length, width and depth of each shape that will be filled with concrete. Sketching these shapes on a piece of paper will help when calculating the total volume of concrete.
Calculate the surface area of your project in square feet. To calculate the square footage of a rectangular area, multiply the length times the width for the area. To find the area of a circle, multiply 3.1416 (pi) by the radius of the circle squared. To find the area of a triangle, multiply the height of the triangle by half of the base.
Multiply the surface area by the height or depth of the concrete to get the volume of the project in cubic feet of material. For a patio slab measuring 720 square feet, Our example slab will be 4 inches thick. Convert 4 inches into feet and then multiply the surface area in feet by the depth in feet. Four inches equals one third of 1 foot, or .3333 feet. Take 720 feet squared times .3333 feet to arrive at 239.97 cubic feet. This is the volume of concrete contained in the example slab.
Convert the answer in cubic feet to cubic yards, because concrete is generally ordered by the cubic yard. Divide the volume in cubic feet by 27. For the example slab, 239.97 cubic feet divided by 27 equals 8.88 yards of concrete.
Assume an approximate 10% wastage factor when ordering concrete to make sure that you will have enough material to complete your job. In the example, 8.88 cubic yards of material times 110 percent is 9.77 cubic yards of concrete.