One formula
Cubic yards = length × width × thickness in feet, divided by 27. A 20×10 slab at 4 inches: 20 × 10 × 0.333 ÷ 27 = 2.47 cubic yards. That is the entire formula — everything else is judgment about what the formula cannot see.
Overage is not padding
Subgrade is never perfectly flat. Ten percent overage is a common allowance because ordering short mid-pour means a cold joint or an emergency truck, and both cost more than the extra quarter yard.
Bags versus trucks
An 80 lb bag yields about 0.6 cubic feet, so a cubic yard is roughly 45 bags. At 2.7 yards, that is about 122 bags — mixed, placed, and finished by hand. Bag counts are useful for small pads and footings; once the pour is measured in yards, ready-mix usually wins on labor before you even compare material price.
Remember what volume leaves out: base prep, forms, reinforcement, and finish are separate estimate lines with their own labor.