How to Measure a Box: Internal vs External Dimensions

Getting box dimensions right is the difference between a product that fits snugly and one that rattles, crushes, or costs more to ship than it should. The key is knowing when to use internal and when to use external measurements.

Internal vs External Dimensions

Every corrugated box has two sets of measurements, and they are not the same thing.

Internal dimensions are the usable space inside the box — the room your product actually has. Always order by internal L×W×H.
External dimensions are the outside of the box, including the board thickness. They matter for stacking, palletising, and how the box fits alongside others.

The gap between the two comes from the corrugated board itself. Single-wall board adds a few millimetres per side; double-wall adds more. On a small box this is minor, but across a full pallet it adds up quickly.

How to Measure Each

Measure in this order, with the box open and flat-based:

Dimension	Internal	External
Length (L)	Longest side, inside edge to inside edge	Longest side, outer face to outer face
Width (W)	Shorter side, inside to inside	Shorter side, outer to outer
Height (H)	Base to the top of the sidewall, inside	Base to top flap, outside

Always list dimensions as Length × Width × Height. Add a small tolerance so your product slides in without forcing the flaps.

Why the Difference Matters

For fit, work from internal dimensions and leave room for void fill or protective padding. For freight, external dimensions drive how boxes stack on a pallet and what a carrier charges. A box that is a few millimetres too large per side can cost you a full row on the pallet.

FUMERO cartons list capacity, material, colour, dimensions and pack size so you can match the box to your product with confidence. Heavy or pallet orders are automatically offered a freight option, quoted after your order.

Need a hand choosing the right size? Browse our boxes or read our companion guide, how to choose a shipping box size.