Concrete is ordered by the cubic yard (27 cubic feet). The math is just volume — with the right shape formula.
Formulas
- Slab: length × width × thickness (all in feet) = cubic feet
- Column: π × radius² × height
- Cubic yards: cubic feet ÷ 27
Example: 20×10 ft patio, 4 in thick
20 × 10 × (4/12) = 66.7 cu ft = 2.47 cubic yards. In bags: ~45 eighty-lb bags per yard → about 112 bags.
Tips
- Order 5–10% extra — short loads and form bulges happen.
- Slab thickness: 4" for patios, 5–6" for driveways.
Our Concrete Calculator handles slabs, columns and wall footings with bag counts built in.