Why Contractors Always Order Extra Concrete
Contractors typically add 5-10% beyond the calculated volume to account for an uneven subgrade (a base that isn't perfectly level absorbs slightly more concrete than a mathematically flat slab assumes), unavoidable spillage and minor form imperfections during the pour, and the practical difficulty of ordering a small top-up mid-pour if the original estimate runs even slightly short — a shortfall discovered partway through a ready-mix delivery is a far bigger problem than a small amount of leftover concrete.
This buffer isn't padding or overcaution — it reflects genuine, common variables that a pure geometric volume calculation doesn't account for.
Why the subgrade adds real, hard-to-predict volume
As covered in the volume-calculation guide, the standard formula assumes a perfectly flat, level base at the intended thickness — a real subgrade with minor low spots or unevenness fills in with extra concrete you didn't account for in a purely geometric calculation, which is exactly what the waste-factor buffer is meant to cover.
Why running short mid-pour is the scenario everyone avoids
Concrete begins setting relatively quickly once mixed, so a delivery that runs short partway through a pour creates a serious, time-pressured problem — ordering a small amount extra upfront is a far smaller cost than the risk, delay, and potential seam or quality issue caused by a mid-pour shortage.