Dimensional weight, often called DIM weight, is a billing convention carriers use to charge for the *space* a parcel takes up in their network — not just its weight. The formula is the parcel's volume divided by a published "dimensional factor" (currently 139 in cubic inches per pound for most domestic services).
For example, a 12" × 12" × 12" box weighs about 5 lb actual but has a dimensional weight of 1,728 ÷ 139 ≈ 13 lb. The carrier bills you for the higher of actual or dimensional weight — so a 5 lb pillow in that box bills as a 13 lb parcel.
DIM weight exists because carriers' costs are driven more by the cube of a trailer than by the weight in it. A pickup truck full of feathers and a pickup truck full of bricks take roughly the same fuel to move; the trailer fills up by volume long before it hits weight capacity.
DIM matters most for lightweight, bulky shipments — pillows, foam packaging, sneakers in oversized retail boxes. Right-sizing your packaging (using boxes that match the contents) is the single biggest lever a shipper has on DIM-driven cost. L5's rate engine flags DIM-impacted shipments in your billing dashboard so you can see exactly where you're paying for empty space.