cubic yard

The cubic yard is the unit of anything delivered by truck. A standard ready-mix concrete truck carries 8 to 10 cubic yards, a residential dumpster is rated in cubic yards, and topsoil, gravel and mulch are all sold that way. The reason it stuck is scale: a cubic foot is too small to price a driveway and a cubic metre was never adopted in the American trades, so the yard's cube landed in exactly the right size band, roughly the volume of a domestic chest freezer.

The arithmetic trap is the factor of 27. A slab 20 ft by 30 ft at 4 in thick is $20 \times 30 \times \tfrac{1}{3} = 200$ cubic feet, which is 7.4 cubic yards, so you order 8 and allow for waste. Anyone who divides by 3 instead of 27 orders nine times too much concrete. The metric equivalent is friendlier here, since a cubic metre is 1.31 cubic yards and the same slab is 5.7 m³, but North American suppliers price in yards and will quote in yards no matter what the drawing says.