Why the extra quarter yard is the cheap one
Ask a crew what went wrong on a bad pour and you will almost never hear "we ordered too much." You hear that the truck left, the last section came up short, and somebody spent the afternoon mixing bags by hand to close a gap that was never supposed to be there.
It only goes wrong in one direction. Ordering long costs you the price of concrete you did not use. Ordering short costs you a second delivery, a cold joint, or an afternoon.
The margin is smaller than the problem
A 10 by 10 pad at four inches is 1.23 cubic yards of actual concrete. Order exactly that and you are betting the subgrade is perfectly flat, nothing spills, and the last wheelbarrow lands exactly where you planned.
Add ten percent for waste and you are at 1.36. Ready-mix sells in quarter yards, so you order 1.5. The gap between the bet and the margin is about a sixth of a yard — the cheapest insurance on the job.
What "waste" actually covers
It is not sloppiness. It is the corner of subgrade that sits an inch low, the forms that bowed under the first pour, the material left in the chute, and the wheelbarrow you always need and never planned for.
A slab does not care what you calculated. It fills the space that is actually there.
Ten percent is the common allowance. On rough ground, or around hand-dug footings, more.
Coming up short is the expensive outcome
Every option is a bad one. A second short load carries a short-load fee from most suppliers, because they are moving a truck for a fraction of its capacity. Mixing the gap by hand means an 80 lb bag yields 0.60 cubic feet and a 60 lb bag 0.45 — so even a tenth of a yard is a stack of bags and a long hour. And if the first pour has started to set, the joint between old and new is a feature of that slab from now on.
If you are mixing on site
Same pad, no truck: 1.36 cubic yards with waste is about 62 of the 80 lb bags, or 82 of the 60 lb. Worth knowing at the yard rather than halfway through the pour.
Run your own numbers
The concrete calculator does this in about ten seconds — length, width, thickness, waste — and shows the formula and every assumption, so you can disagree with it where your job is different. It is free, there is no sign-up, and it is the same calculator that runs inside the Hut.