Concrete Volume with Waste Allowance

Ready-mix is sold by the cubic yard and delivered in trucks that hold eight to eleven of them, and the driver will not wait while you find a fourth of a yard. Hence the waste allowance: 5 % on a formed slab poured on polyethylene, up to 10 % on footings dug in soft ground where the trench over-excavates itself, and more on a rough-graded subgrade. The 27 in the conversion is the whole game — a cubic yard is 27 ft³ — so a 6 in slab is 0.5 ft thick and 54 ft² of it makes exactly one yard.

A worked example: a 40 ft × 30 ft driveway at 6 in thick is 40 × 30 × 0.5 = 600 ft³ = 22.2 yd³ neat; with 5 % waste, order 23.3 yd³, which in practice means calling for 23.5 yd³ since most suppliers sell in quarter-yard steps. The classic error is entering thickness in inches alongside feet — formula.expert converts for you, but on a napkin a 6 becomes a 12-fold overpour.