Concrete comes by the cubic yard or cubic metre from a truck, or by the bag, and the arithmetic is the same either way: length × width × thickness for every slab, footing and wall; the area of a circle times the depth for each post hole or column; and, for solid steps, one rise under the first tread, two under the second, and so on. This page adds up as many pours as the job has, each under a name of its own, and keeps them all in its link.
What catches people out is ordering exactly what the forms hold. The National Ready Mixed Concrete Association advises ordering 4 to 10 per cent more, for waste, forms that bow, ground dug a little deep and slabs a little thick — a slab an eighth of an inch thicker than planned at 4 inches is already 3 per cent short. The page starts at 10 and offers 4; the finished order shows both what the forms hold and what to order.
Bags are counted at the yields their makers print. Quikrete’s Concrete Mix makes 0.30 cubic feet from a 40 lb bag up to 0.675 from a 90 lb one — 0.6 from the common 80 lb bag — and is made for pours 2 inches thick or more, so the page says so when a slab is thinner. In Britain, a 20 kg bag of Blue Circle Multi-Purpose Concrete makes about 0.01 m³.
Rooms measured on the calculator or drawn in the floor plan editor come across as slabs of their own area, so an L-shaped patio does not have to be cut into rectangles first. Deck footings arrive from the deck calculator as round pours. For gravel, soil or sand, the cubic yard calculator adds up sections the same way.