A pricing method carriers use to charge for package size, not just actual weight, based on volume relative to weight.
Dimensional weight (DIM weight) is a billing calculation carriers like UPS, FedEx, and USPS use to charge for low-density, high-volume packages. It estimates a "weight" based on a package's cubic size (length × width × height ÷ a carrier-specific divisor), and carriers bill based on whichever is greater — actual weight or dimensional weight.
The practice exists because bulky-but-light packages take up valuable space on a truck or in a plane without weighing much, so carriers price for the space consumed rather than just the scale weight. A box of pillows might weigh 3 lbs but be billed as if it weighs 12 lbs if its dimensional weight is higher.
For eCommerce brands, DIM weight pricing makes packaging optimization a direct cost lever. Right-sizing boxes, using lower-divisor carriers, and avoiding excess void fill can meaningfully cut shipping costs — which is why many 3PLs offer automated box-sizing and packaging optimization as part of their pick-and-pack service.
A 16"x12"x10" box of lightweight foam inserts weighs 4 lbs but has a dimensional weight of 16 lbs at a 120 divisor — the carrier bills for 16 lbs, not 4.
Multiply length × width × height (in inches), then divide by the carrier's DIM divisor (commonly 139 for domestic UPS/FedEx ground, though this varies by carrier and service). Compare the result to actual weight — the carrier bills whichever is higher.
Use right-sized packaging instead of oversized boxes, minimize void fill, and ask your 3PL whether they offer automated box-selection during pack-out — even small reductions in box volume can lower shipping costs at scale.
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