On June 23, 2019 the USPS made changes to how it applies Dimensional Weight to Priority Mail Packages. The two main changes are that Dimensional Weights can now be applied across all USPS postal zones and that Dimensional Weight Calculation is now less favorable for shippers.
Dimensional Weight or DIM is a formula that is used to calculate the weight of a box based on the dimensions instead of the physical weight. This is applied so large boxes with light content get charged on their dimensional weight to take into account the volume they are taking up when the carriers transport them. The formula for DIM is (length inches x height inches x width inches)/166. Previously the formula was (length inches x height inches x width inches)/194.
Previously USPS Dimensional Weight only applied to shipments being sent to USPS zones 5-9. With the USPS Dimensional Weight changes the DIM applies to shipments in all zones.
The good news is that First Class Mail still only uses weight based rating and for Priority Mail DIM rating only applies to shipments that are 1 cubic foot (or essentially have a DIM of 10.41). This is essentially anything with more volume than a 12 inch by 12 inch by 12 inch box.