How it works
Cubic feet = Length × Width × (Thickness ÷ 12) · Cubic yards = Cubic feet ÷ 27 · Cost = Cubic yards × price per yard
An 80-lb bag yields about 0.6 cubic feet. For anything over ~1 yard, ready-mix is usually cheaper than bags.
Example
A 10 × 10 ft slab at 4 in thick is about 1.23 cubic yards — at $150/yard, roughly $185.