How it works
Cubic feet = Length × Width × (Thickness in inches ÷ 12) · Cubic yards = Cubic feet ÷ 27
An 80-lb bag yields about 0.6 cubic feet of mixed concrete.
Example
A 10 ft × 10 ft slab at 4 in thick is about 1.23 cubic yards (~56 bags) — at that size, ready-mix is usually cheaper than bags.