Many quotation mistakes start with one word: dimensions. The same box has two sizes, an internal one and an external one, and confusing them leads to boxes that are too tight for the product or too big for the carrier. This article explains how to specify a box and how to work backwards from a size limit.

The convention: internal length × width × height

Box manufacturers normally quote a box by its internal dimensions in millimetres, in the order length × width × height. Length and width describe the opening, and height is the depth of the box. When you send an enquiry, write “internal” next to the numbers, together with the FEFCO code or a drawing.

Why the external size is different

The board has thickness, so the outside of the box is larger than the inside. As a rough guide, the external size is the internal size plus twice the board thickness in every direction. Thin flutes such as E add a little, and thicker board (for example B, C, BC, or 5- and 7-ply) adds more. The exact difference depends on the board and the construction, so ask us for it when the limit is tight.

When the external size decides

• Parcel lockers: compartments are described by their maximum parcel size. See boxes for parcel lockers.
• Mailbox parcels: a thickness limit of about three centimetres leaves very little room for board.
• Pallets: the external size decides how many boxes fit on a layer.
• Carrier limits: sums of dimensions and longest side are measured on the outside.

Working backwards from a limit

1. Take the maximum external size allowed by the carrier or the compartment.
2. Subtract twice the board thickness in each direction.
3. Leave a margin of a few millimetres, because a full box can bulge.
4. Check that the product plus any insert fits into the resulting internal size.

The margin is our practical advice, not a carrier requirement, so always test a real packed parcel. If the product needs protection inside, see corrugated inserts and dividers.

What to send in an enquiry

• the internal dimensions (L × W × H, mm) and the FEFCO code or a drawing,
• the board (3-, 5- or 7-ply) or the weight of the packed product,
• any external size limit from a carrier, locker or pallet plan,
• printing requirements and the quantity per size.

We make boxes to size, from one pallet, and short runs without tooling are possible for straight-cut formats: see short runs without tooling costs. Read the guide to buying cardboard boxes from Poland, how to choose corrugated boxes in bulk and our wholesale terms, and send your enquiry through our home page.

Frequently asked questions

Which dimensions do manufacturers usually quote?

Internal dimensions, in millimetres, in the order length × width × height. State clearly that they are internal when you send an enquiry.

How much bigger is the external size?

It depends on the board. Roughly, the external size is the internal size plus twice the board thickness in each direction, and thicker board such as 5- or 7-ply adds more than a thin single-wall board. Ask for the exact figure for your specification.

How do I find the internal size for a locker or mailbox limit?

Start from the maximum external size, subtract twice the board thickness, and leave a few millimetres of margin because a full box can bulge. Test the result with a real packed parcel.