Ordering concrete is one of those jobs where the math matters more than the muscle. Ready-mix is sold by the cubic yard, delivered by trucks that carry about ten yards each, and priced with short-load fees when you order small. Underestimate and the crew stands around waiting for a second truck; overestimate and you have paid for concrete that hardens in the drum. The good news is that the calculation itself is simple — volume, plus a sane waste factor.
Start with the shape. For a rectangular slab, the volume in cubic feet is length × width × thickness, all in feet. A 20-by-12-foot patio slab at 4 inches thick (one-third of a foot) is 20 × 12 × 0.333 = 80 cubic feet. Since one cubic yard equals 27 cubic feet, divide by 27: 80 ÷ 27 ≈ 2.96 cubic yards. That is the theoretical volume — the amount the geometry demands, before reality intervenes.
Reality always intervenes. Forms flex outward under the weight of wet concrete, subgrades are never perfectly level, and some material is lost in the wheelbarrow, the chute, and the cleanup. The industry-standard allowance is 5–10% extra, and the Measurely Concrete Slab Calculator applies a stated waste factor you can see and adjust. On that 2.96-yard patio, a 10% allowance brings the order to about 3.26 yards — and because suppliers round up to the quarter or half yard, you would order 3.5 yards.
Footings, columns, and stairs need their own shapes. A round column footing is π × radius² × depth; a rectangular wall footing is length × width × depth, just like a thin slab. The Concrete Calculator handles slabs, walls, footings, columns, and stairs in one place, converting each shape to cubic yards automatically. When a project mixes shapes — say a slab plus a thickened edge footing — calculate each part separately and add the yardage together before applying the waste factor once.
For small jobs, pre-mix bags beat the ready-mix truck. An 80-pound bag of concrete mix yields roughly 0.60 cubic feet of cured concrete (a labeled rule of thumb — check your bag, since yields vary slightly by brand). Our 80-cubic-foot patio would need 80 ÷ 0.60 ≈ 134 bags, which is why anything over about 2 cubic yards usually justifies a truck: 134 bags is a full day of mixing and a sore back. The Concrete Bag Calculator runs this in reverse — enter your volume, pick 40, 60, or 80-pound bags, and get an exact bag count.
When you call the supplier, have three numbers ready: the total cubic yards (with waste included), the slab thickness and any special requirements (fiber reinforcement, air entrainment for freeze-thaw climates), and your pour date. Ask about the short-load fee threshold — typically around 4–5 yards — because ordering 3.5 yards sometimes costs nearly as much as ordering 5. If two small projects are happening the same week, combining them into one delivery can pay for itself.
A final checklist before you order. Measure twice, in feet and inches, and convert inches to decimal feet (4 inches = 0.333 ft, 6 inches = 0.5 ft). Add the waste factor after summing all shapes, not before. Confirm the truck can reach the pour — long chutes and pump trucks cost extra. And schedule the crew for the delivery time, not the other way around: concrete waits for no one.