Concrete is generally measured in cubic footage, using a volume formula of length x width x thickness. The volume measurement is needed because concrete surfaces of the same coverage area can differ in the amount of actual concrete used to create those surfaces, depending on how thick the slab is.
Cubic footage is used in measuring concrete because concrete mix is often sold using cubic feet. Concrete mix may also be sold by the cubic yard. To convert between cubic feet and cubic yards, divide the total footage by 27, rounding up the result to ensure that enough mix is purchased.