Dimensional weight, also known as volumetric weight, is a pricing technique for commercial freight transport, which uses an estimated weight that is calculated from the length, width and height of a package. 

It is important to note, higher of actual or volumetric weight is considerd for calculating the shipping charges.

For example - 

A. For shipping in Express mode - A box having an actual weight of 1kg and package dimensions of 40cm, 35cm and 30cm would have volumetric weight of (40 * 35 * 30)/5000 = 8.4 kgs for Express / Air shipments.

B. For shipping in Surface mode - A box having an actual weight of of 1 Kg and package Dimension: 40cms, 35cms, 30cms, volumetric weight of (40 * 35 * 30) / 4500 = 9.3 Kgs for surface shipments