How this concrete calculator works
Concrete volume equals length × width × depth. Enter your project dimensions in feet and the slab thickness in inches, and the tool converts everything to cubic feet first, then to cubic yards (divide by 27 — ready-mix is sold by the yard) and to the number of bags you would buy at a hardware store.
Most flatwork like driveways, patios and shed slabs is poured 4 to 6 inches thick. Footings usually need more depth, and fence post holes are calculated per hole. A 10% waste allowance is a sensible default: it covers uneven ground, spillage and forms that are not perfectly square.
Handy rules of thumb
- 1 cubic yard = 27 cubic feet = about 45 bags of 80 lb mix.
- 4-inch slab: one cubic yard covers about 81 square feet.
- 6-inch slab: one cubic yard covers about 54 square feet.
- For small jobs (under 1 cubic yard) bags are usually cheaper; for larger pours, ready-mix delivery is faster and cheaper per yard.