Volumetric weight...
sometimes called "dim" or dimensional weight, is based on volumetric standards. When the dimensional weight exceeds the actual weight the dimensional weight is used to determine the charges.

You should calculate volumetric weight on every shipment, compare that weight to its actual weight, and use the greater of the two to determine your shipping costs. International volumetric weights are calculated using the international standards formula below.

Volumetric Charging: When shipping lightweight, bulky packages,
ZipX uses the following formula to calculate volumetric weight.

Metric Inches/Pounds Length:  Width:  Height:  Dim. Weight: