Concrete volume needed for a slab or footing, plus how many pre-mix bags that takes.
Slabs and footings are usually poured a few inches thick, so inches give a more precise input. The calculator converts thickness to feet internally (dividing by 12) before multiplying by length × width to get a volume in cubic feet.
Pre-mix concrete bags commonly come in 80 lb (yielding about 0.6 ft³ per bag) and 60 lb (about 0.45 ft³ per bag) sizes. The calculator divides your total volume by each bag yield and rounds up, so you can buy whichever size is available.
The waste allowance covers spillage, over-excavation, uneven subgrade, and minor measurement error — common on real pours. The "Recommended To Buy" numbers multiply your raw volume and bag counts by 1.1 so you don't come up short mid-pour.