Concrete is sold in cubic yards, so the number of square feet in that yard depends upon how deep the concrete is poured. Concrete can be poured in different amount of thickness.
For instance, if the slab being put in is 4 inches thick, one cubic yard of concrete covers 81 square feet. If it is 8 inches thick, a cubic yard fills 41 square feet. If the slab is 1 foot thick, a cubic yard covers only 21 square feet. Concrete spills, and the surface being filled is sometimes uneven. To be sure that there's sufficient concrete for the job, purchase approximately 10 to 15 percent more concrete than what is expected.