Dimensional weight (DIM weight) is a pricing technique used by shipping carriers that factors in the size of a package, not just its actual weight. If a package is large but light (like a pillow), the carrier charges based on dimensional weight instead of actual weight — because the package takes up valuable truck/airplane space.

Carriers compare actual weight vs dimensional weight and charge based on whichever is greater — the "billable weight." This is why optimizing your box size directly reduces shipping costs.

• FedEx: 139 (domestic), 139 (international)
• UPS: 139 (all services)
• USPS: 166 (Priority Mail, Priority Mail Express)
• DHL: 139 (Express)

Lower divisor = higher dimensional weight = higher shipping cost. USPS has the most favorable divisor (166), making it the cheapest option for large, lightweight packages.

• Right-size your boxes: Use the smallest box that safely fits your product. Even 1 inch of excess in each dimension adds up
• Use poly mailers: For soft goods (clothing, accessories), poly mailers collapse to the product's size — no wasted space
• Custom box sizes: If you ship high volume, custom boxes that fit your top products exactly can save thousands
• Negotiate with carriers: High-volume shippers can negotiate lower DIM divisors (higher numbers = lower DIM weight)
• Compare carriers: USPS uses a 166 divisor vs 139 for FedEx/UPS — up to 19% lower DIM weight