Inside vs outside box dimensions
The same box has two sets of dimensions. The inside dimensions describe the space available for your product. The outside dimensions describe how much room the box takes up. Box makers quote one, carriers charge on the other, and the gap between them is the thickness of the corrugated board.
Why box makers quote inside dimensions
A box exists to hold something, so the number that matters when you order is the space inside. When you ask a supplier for a 12 × 10 × 8 box, you get a box with 12 × 10 × 8 inches of usable space. Stock box catalogs work the same way.
Why carriers measure the outside
A truck or plane is filled by the outside of the box. UPS, FedEx, USPS and freight carriers all measure the outside, round each side, and use those numbers to calculate dimensional weight and to check size limits.
How much bigger is the outside?
For a regular slotted container, the most common shipping box style:
- Length and width each gain two wall thicknesses, one on each side.
- Height gains roughly four thicknesses, because the top and bottom each have two layers of flaps folded over each other.
| Board | Wall thickness | 12 × 10 × 8 in box, outside |
|---|---|---|
| E-flute | about 1/16 in | about 12⅛ × 10⅛ × 8¼ in |
| B-flute | about 1/8 in | about 12¼ × 10¼ × 8½ in |
| C-flute | about 5/32 in | about 12⅓ × 10⅓ × 8⅝ in |
| BC double wall | about 1/4 in | about 12½ × 10½ × 9 in |
These are approximations. Actual board caliper depends on the mill and grade, and manufacturers add small allowances at the folds.
Why a quarter inch matters
Carriers round measurements to whole inches. A box that is 12 inches inside and 12¼ outside can be measured as 13 inches. Apply that to all three sides and the dimensional weight changes noticeably.
Take a 12 × 12 × 12 inside box in C-flute. The outside is about 12⅓ × 12⅓ × 12⅝. Rounded up to 13 × 13 × 13, that's 2,197 cubic inches, or 16 pounds at a 139 divisor. The same box calculated from its inside size would come to 1,728 cubic inches, or 13 pounds. That's three pounds of difference on every shipment from board thickness alone.
Which number should you use?
- Ordering boxes: inside dimensions.
- Checking whether a product fits: inside dimensions.
- Shipping quotes and labels: outside dimensions.
- Pallet and container planning: outside dimensions.
If a supplier's listing doesn't say which it is, ask. Some retail product listings quote outside sizes, which can leave you with a box slightly too small for your product.