How Do You Figure How Much Concrete to Use?
Figure out the amount of concrete to use by calculating the volume of the desired concrete slab and the footings supporting the slab, then converting this area to cubic yards. The method for calculating the volume of concrete needed for slabs is also used for any concrete structure.

Calculate the volume of the desired slab
Multiply the desired length of the slab by the width and the slab depth in feet. The result is the volume of the slab in cubic feet. Divide this amount by 27 (there are 27 cubic feet in a cubic yard) to determine the volume of the slab in cubic yards.

Calculate the volume of the footings
Calculate the volume of each of the footings of the slab in feet using the same volume calculation. Convert the volume of each footing into cubic yards, and then add the total for each footing together to get the total volume for the footings in cubic yards.

Determine the amount of concrete needed
Add the volume of the slab to the volume of the footings to calculate the total amount of concrete necessary for the job. Order concrete from a supplier or purchase enough concrete bags to fill the desired space after preparing the hold for the concrete pour.