At one time, shipping prices were determined solely by the weight of the box. A heavier box costs more to ship than a lighter box. Sure, that makes sense.
But a few years back all the shipping companies added another factor - the size of the box. There was a realization that a large box uses more space inside a truck or plane. Thus, a new factor was added to calculate shipping prices called "dimensional weight."
Now all the shipping companies consider the dimensions of the box in the shipping calculations. If you go to FedEx or USPS they take out their measuring tapes to find the length, width, and depth of the box. This is put into their computer along with the actual weight. The shipping calculation will take the heavier of the two, either the actual weight or the dimensional weight.
To save time, Kim Cridler recommends writing the dimensions of the shipping boxes on the outside of the box.
The dimensional weight is a shipping factor that you can not avoid, but it is something to consider when you double box your artwork for shipping. A one-inch to two-inch margin between the interior and exterior shipping box is important to protect most artwork. So unless you have something unusual, any larger-sized outer box may be costing you more money for shipping.
This post was updated on June 17, 2022, to provide current links.