How to Measure a Cubic Foot
Surface area of a concrete slab or sand base is directly affected by its thickness. In the photo you see two examples of a measured cubic foot. Since a cubic foot is measured by Length x Width x Thickness, a cube 1 foot long x 1 foot wide x 1 foot thick is therefore 1 cubic foot.
As you reduce the thickness, you increase the surface area (LxW) coverage, like cake batter.
To determine how many cubic feet you need for a hole, measure the length, width and thickness together, all measured in feet. So a 4" thickness is 4"/12" or 0.333 feet. The result of this Length x Width x Thickness multiplication is cubic feet.
Likewise, a cubic yard (yard = 3 feet) is a cube 3 feet by 3 feet by 3 feet, or 27 cubic feet.
Here's a helpful table that shows how surface area changes as thickness varies for one cubic yard (27 cubic feet) of material: