FedEx, UPS, and USPS dimensional weight formulas all follow the same basic guidelines: Determine the package’s dimensions, multiple them to get the volume, and divide the volume by the carrier’s DIM Factor.
This math will provide you with a DIM weight. The cost to ship your package is based on whichever is greater between the DIM weight and actual weight. A package with a dimensional weight of 8 pounds and an actual weight of 5 pounds would ship as an 8-pound parcel. If a package with the same dimensions had an actual weight of 10 pounds, the shipping charge would be based on the actual weight of 10 pounds.
Generally, packages that are relatively heavy and small use actual weight more often. Large or lightweight packages are more likely to use DIM weight because of their high volume. Imagine the large box for shipping a light titanium bike frame in a single piece.
FedEx, UPS, and USPS will always use the larger value between DIM Weight or Actual Weight as the final Billed Weight.