USPS applies dimensional weight to Priority Mail packages larger than 1 cubic foot (1,728 cubic inches). A large box with a light item inside can cost more than its actual weight suggests.

Since July 12, 2026, USPS rounds each side up to the next whole inch, multiplies length × width × height and divides by 139 (previously 166). If that result is higher than the actual weight, it becomes the billable weight.

For example, a 12 × 12 × 14 inch box has 2,016 cubic inches. Divided by 139, that rounds up to a dimensional weight of 15 lb. If the packed box weighs 4 lb, you pay the 15 lb rate.