Cross-section area
12 sq units
Length (layers)
5 units
Volume
60 cu units
Volume = cross-section × length = 12 × 5 = 60 cubic units
Layers:
The end face is the cross-section — here it holds 12 unit squares, so one layer of cubes is 12 cubes. Stack one identical layer for every unit of length, and the whole box is just cross-section × length cubes.