Concrete is ordered by volume in cubic yards — and the key conversion is that one cubic yard equals 27 cubic feet. Ready-mix suppliers sell by the cubic yard with a minimum order (often 1 yard, with short-load fees below it), while bagged concrete from the hardware store is measured in cubic feet per bag: an 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45. The volume formula is length x width x thickness, all in feet, divided by 27 to get cubic yards. Thickness must be in feet too — a 4-inch slab is 4/12 = 0.333 feet, the conversion people most often fumble.
Standard thicknesses: 4 inches for patios, walkways, and shed floors; 5 to 6 inches for driveways; 6+ inches for heavy loads or frost-prone regions with proper reinforcement. Always add a waste and uneven-subgrade allowance of 5 to 10 percent — excavated ground is never perfectly level, and concrete cannot be returned once the truck arrives. For bagged concrete, divide the cubic footage by the per-bag yield and round up; bags are cheap compared to a second trip.
Ordering slightly over is the professional norm because a short load means a cold joint — a visible seam where fresh concrete meets concrete that already started setting — which weakens and discolors the slab. Round fractional yards up to the nearest quarter yard when ordering ready-mix. And remember: this calculator estimates volume only; structural slabs, footings, and anything load-bearing need an engineer's specification, not a web estimate.