Cubic yards and bags for a rectangular slab. Change the numbers and the answer updates; the starting values are only an example.
How it is worked out
- Volume in cubic feet is length times width times thickness, with the thickness divided by 12 to turn inches into feet.
- Cubic yards are cubic feet divided by 27, because one cubic yard is 3 ft by 3 ft by 3 ft.
- The extra percentage is added on top, and the bag count rounds up to whole bags.
Worked example
With the starting values, a slab 12 ft by 10 ft and 4 inches thick is 40 cubic feet, which is 1.48 cubic yards, or 1.63 cubic yards with 10% extra. At 0.6 cubic feet per bag that is 74 bags.
Good to know
- Ready-mix trucks are ordered in cubic yards; bags suit small pours.
- Measure the thickness at the thickest point if the ground is uneven.
- Fields marked as read from the label have no built-in value you should trust: products differ, so use the number on yours.
Concrete by slab size
Common sizes are worked out on their own pages: 10 by 10 ft, 10 by 12 ft, 12 by 12 ft, 12 by 16 ft, 12 by 20 ft, 16 by 16 ft, 20 by 20 ft, 24 by 24 ft.