The amount of concrete needed for a 10-by-20-foot slab depends on the thickness of the slab. A 10-by-20-foot slab with a thickness of 4 to 8 inches requires 2.5 to 5 yards of concrete.
Calculate the amount of concrete needed by determining the volume of the desired slab. The recommended slab thickness depends on what the slab is for. Driveway slabs should have a thickness of between 6 and 8 inches, and sidewalks and barn floors should be 5 to 6 inches thick. Porches and home garage floors should be 4 to 5 inches thick, and farm building and home basement floors should be 4 inches thick.