How to measure a mailer box so your product does not rattle

Nearly every packaging problem we are asked to fix comes down to the same thing. Not the print. Not the board. The dimensions. A box that is 10mm too generous lets the product slide on every bump of the courier network; a box that is 5mm too tight will not close without a fight at the packing bench.

Boxes are measured internally, not externally

When you order a custom mailer box, the three numbers you give are the internal dimensions — the usable space inside once the box is folded and glued. This trips people up constantly, because the box you are holding and measuring with a tape is an external measurement.

The difference is the board itself. Single-wall corrugated is roughly 3mm thick, double-wall closer to 7mm. On a small box, quoting external dimensions by mistake means losing 6mm of internal width — enough to make a snug product unpackable.

The order is always length × width × height. Length is the longest side of the opening, width is the shorter side, height is the depth from base to lid.

Measure the product, then add clearance

Start with the item going inside, at its widest point in each direction. Then add clearance:

• Rigid products (a bottle, a book, a boxed device) — add 3 to 5mm on length and width.
• Soft or irregular products (apparel, cosmetics with a pump, anything that bulges) — add 8 to 10mm, and measure the item compressed the way it will actually sit.
• Multiple items — lay them out in the arrangement they will ship in and measure the whole footprint, not each piece.

Height needs slightly more thought. A mailer box lid folds down and tucks; if the contents sit proud of the walls, the lid bows and the front tab will not seat. Leave 3 to 5mm of headroom below the top edge.

Where void fill fits in

If you plan to use tissue, crinkle paper or a foam insert, that material needs its own space. A common mistake is sizing the box perfectly to the product, then discovering there is nowhere for the tissue to go. Decide on your internal presentation first, then size around the finished arrangement.

Better still, size the box tightly and use a fitted insert instead of loose fill. An insert holds the product still, costs less than the volume of paper it replaces, and makes the unboxing feel deliberate rather than padded.

The economics of one extra centimetre

Box dimensions drive two costs that have nothing to do with the box itself.

Board yield. Boxes are cut from standard sheets. A design that is a few millimetres over a threshold can drop the number of boxes per sheet from six to five — a 17% jump in material cost for a change no customer will ever notice.

Dimensional weight. Carriers bill on whichever is greater: actual weight or volumetric weight. An oversized box on a light product means paying to ship air, on every single order, forever. Trimming 2cm from a box shipping 500 units a month is real money.

This is why we send a dieline with every quote. If your dimensions are sitting just over an efficient threshold, we will tell you before anything is printed.

A quick checklist before you order

1. Measure the product itself, at its widest point in each direction.
2. Add clearance — 3 to 5mm rigid, 8 to 10mm soft.
3. Add space for tissue or an insert if you are using one.
4. Confirm the numbers are internal, in L × W × H order.
5. Ask for a physical sample before committing to a full run.

That last step is the one worth insisting on. A sample costs a fraction of a production run, and it is the only way to know a box fits — a screen cannot tell you that.