How concrete math works
Concrete is sold by the cubic yard: 27 cubic feet. Multiply length by width by thickness (in feet), divide by 27, and add waste. Ten percent waste covers uneven ground, spillage, and forms that are not perfectly square — skip it only on a small, perfectly formed job.
Slab thickness guide
- 3.5–4 inches: walkways, patios, shed floors.
- 4–5 inches: driveways for cars.
- 5–6 inches: driveways for heavier vehicles, garage floors.
Frequently asked questions
How many 80 lb bags make a yard?
About 45 bags at 0.6 cubic feet each. For anything over about half a yard, ready-mix delivery is usually cheaper and far faster than mixing bags.
Should I order extra?
Yes — 10% is the standard allowance. Running short mid-pour leaves a cold joint, which is a weak, visible seam.