What Is Dimensional Weight?
Dimensional weight (DIM weight) is the weight a carrier assigns to the space your package occupies — not how heavy it is. It exists because space on planes and trucks is limited: a truck full of feather-light pillows is as full as a truck full of bricks.
The formula
DIM weight = length × width × height ÷ divisor. The divisor is the density the carrier assumes: 139 in³/lb means one pound of capacity for every 139 cubic inches of volume. A lower divisor charges more for the same volume.
- FedEx: 139 (US domestic)
- UPS Daily: 139 · UPS Retail: 166
- USPS: 139, applied only above one cubic foot
- DHL Express: 5,000 cm³/kg
Rounding matters more than you think
FedEx, UPS, and USPS round every dimension up to the next whole inch before multiplying. A box measured at 47.1 × 20.1 × 10.1 in is billed as 48 × 21 × 11 in — 11,088 in³ instead of 9,553 in³, a 16% jump that can cross surcharge thresholds on its own. Measure to the fraction, and know what the carrier will round to.
Billable weight
Billable weight is the greater of actual weight and DIM weight. If your box is big relative to its weight, DIM wins and you pay for the box. If your product is dense, actual weight wins and the box size barely matters — until it crosses a surcharge threshold.