The amount of concrete that comes from a 60-lb. bad of pre-mixed concrete depends on what depth in which the slab is being poured. The Quikrete concrete calculator can help builders determine how much concrete they'll need to perform a given job based on the size of the slab desired.
Concrete poured for foundations is typically poured a level or close to level surface in what is called a slab. This slab provides stability for the structure which will be built atop it, resisting moisture and erosion.
For example, 5 square feet with a 4-inch slab requires two 60-lb. bags of pre-mixed concrete. This high demand makes pouring a foundation an expensive proposition for builders.