How the volume is worked out
For a slab, footing or wall the calculator multiplies length × width × depth, converting the depth from inches to feet first, so a 10 × 10 foot slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet. For a round column, sonotube or post hole it uses the cylinder formula π × radius² × height, with the radius taken as half the diameter in feet. The single-pour volume is multiplied by how many you are pouring, then the waste percentage is added. Divide cubic feet by 27 to get cubic yards, which is how ready-mix is ordered.
Bags or a truck
Premix bags are sized by weight but sold to fill a volume. An 80 lb bag of standard concrete mix yields about 0.6 cubic feet, a 60 lb bag about 0.45 and a 40 lb bag about 0.3; the exact yield is printed on the bag and varies a little by brand. The calculator rounds each bag count up, because you cannot buy part of a bag. As a rough guide, anything over a yard and a half is easier and often cheaper as ready-mix, even with a short-load fee, and anything under half a yard is easier in bags. Enter your supplier's price per yard to see the delivered cost next to the bag counts.
Why add waste
Real pours always take more than the drawing. The sub-grade is never perfectly flat, forms bow, some concrete stays in the mixer and the wheelbarrow, and running short halfway through a pour leaves a cold joint. 5 to 10% is normal for slabs on a well-prepared base; use 10 to 15% for footings dug into soil, where the trench is rarely the exact size.
What this does not cover
The result is concrete volume only. Gravel base, forms, rebar or wire mesh, expansion joint strips, curing compound and finishing tools are separate. Steps, tapered footings and irregular shapes can be estimated by splitting them into rectangles and running each one.