From slab size to volume
Concrete is sold by volume, so the calculation turns the surface area of your slab into a volume using its thickness. Because thickness is usually given in inches, it is divided by 12 to convert to feet before multiplying, then the result is divided by 27 to express cubic feet as the cubic yards used for ready-mix orders.
Ordering with confidence
Measure carefully, round up, and add a margin for spillage and subgrade variation. For bagged concrete, the calculator rounds up to whole bags so you are never short. For larger pours, compare the cost of bags against a ready-mix delivery, which usually wins once you pass half a cubic yard.