How Do You Measure A Box -
Understanding a Boxes Length, Width, and Depth
Box sizes are measured by using inner dimensions unless otherwise specified. The size of your contents actually determine the inner box dimensions. Domestically (within the United States), those dimensions are listed in the following sequence:

Length (L) x Width (W) x Depth (D)*

Internationally, the dimensions are stated as Length (L) x Breadth (B) x Height (H)*

Dimensions are based on the opening of an assembled box. Looking at the opening, the longer of the two sides is considered the "length." The shorter of the two sides is the "width." The side perpendicular to the length and width is the "depth" of the box.

*Exceptions are bookfolds, bin boxes & divider bins where "width" precedes "length" & "depth" of the measurement sequence.