Cubic box measurement or volume is calculated by multiplying the length, width and height of a box.  Standard industry moving boxes are commonly measured and labeled in cubic feet (divide by 1728 to convert from cubic inch to cubic feet)
For example:
Small Moving Box
16 3/8" x 12 5/8" x 12 5/8" = 2610/1728 = 1.5 cu. ft.
Medium Moving Box
18 1/8" x 18" x 16" = 5220/1728 = 3.0 cu. ft.