Dimensional weight calculator or DIM is a conceptual weight system used by the transportation companies worldwide.
Actually historically speaking shipping costs had been calculated based on the gross weight of the consignment either in kilograms or in pounds. But this created problems from the transportation companies. By charging for weight, it became unprofitable for these companies to carry light weight packages. So the concept of Dimensional weight calculator was brought forth.
It is the weight of the package at a minimum density chosen by the freight carrier. One just needs to provide the longest dimensions for length, width and height and the calculator will do the rest.
The freight company takes the larger of the two weights, the gross weight and the dimensional weight to calculate the weight of the package for billing purposes.