How to calculate concrete for a slab
Concrete is ordered by volume. Multiply length by width by thickness to get cubic feet, then divide by 27 to get cubic yards:
- Cubic feet = length (ft) × width (ft) × thickness (ft). Thickness in inches divided by 12 gives feet.
- Cubic yards = cubic feet ÷ 27.
Then add a waste allowance so you don't come up short mid-pour. This calculator applies your waste percentage to both the yardage and the bag counts.
Bags vs ready-mix
An 80 lb bag of concrete yields about 0.60 cubic feet; a 60 lb bag about 0.45. For anything over roughly a cubic yard (about 45 to 60 bags), a ready-mix truck is almost always cheaper and far less labor than mixing bags by hand.
Bag yields vary by mix and brand. Check your bag, and always confirm the truck minimum with your supplier before ordering.