Choose a carton by its internal dimensions, measured from the product plus whatever protection sits around it - never from an external size. Then check the resulting external dimensions against your carriers' size bands, because a carton that fits the product perfectly but sits just over a threshold costs more on every order.
Internal, not external
The single most common error in carton specification is quoting or ordering on external dimensions. Board has thickness - a double wall carton noticeably more than a single wall one - so an external measurement tells you nothing reliable about what will fit inside.
Specify internal, always, and state it as internal. It removes an entire category of disappointment when a pallet arrives.
Build the internal size from the product outward
- Measure the product's real external dimensions - the actual item, not the drawing.
- Add the inner protection: insert thickness, wrap, interleaving, whatever sits between product and wall.
- Add a working tolerance so the packer is not fighting the box.
- Consider orientation - which way the product sits decides which dimension is critical.
- For multi-item orders, decide whether the carton holds one or several, and specify accordingly.
That gives the internal dimension. Only then do you look at the external, which is what the carrier measures.
Then check the external against the bands
This is the step that converts a well-fitting carton into an economical one. Parcel pricing is banded, so a carton whose external dimensions sit just over a threshold costs a step change on every order it carries.
If the external size lands just over a band, it is frequently worth revisiting the internal specification - a slightly thinner insert, a different orientation, or a different board construction can bring it back under. See postal and parcel packaging.
How many sizes to hold
Carton size selection is not one decision; it is a ladder design. The tension is real: more rungs fit better, fewer rungs pack faster and tie up less space.
| Approach | Result | When it is right |
|---|---|---|
| One size for everything | Fast, simple, ships a lot of air | Very narrow product range |
| A rung per product group | Good fit, manageable count | Most businesses |
| A rung per SKU | Perfect fit, unworkable operationally | Almost never |
| Rungs placed at carrier bands | Good fit and good carriage | The target |
Designing the ladder.
Plot your SKUs by their largest dimension and look for clusters rather than spacing rungs evenly. See packaging for fulfilment.
Board construction is part of the size decision
Wall construction affects both the internal-to-external relationship and how much the carton can take. Double wall takes more external space for the same internal volume - which can push a pack over a band.
Specify the board for the weight inside and the load that will sit above it in a stack, then check what that does to the external dimensions. See packaging materials.
When no standard size works
If your product clusters do not line up with available stock sizes - which is common for anything that is not roughly cuboid - a made-to-size carton removes the compromise entirely.
It needs a run quantity to justify, and that run has to live somewhere, but it removes the fill and the air from every order. See bespoke carton manufacture and packaging stockholding.
A checklist before you order
- Internal dimensions stated as internal, with tolerance.
- Board construction specified for the stack, not just the contents.
- External dimensions checked against every carrier you use.
- Fill ratio calculated and acceptable.
- Format and closure chosen for how the pack is built at the bench.
- Sample approved with your real product before a production run.
The short version
- Specify internal dimensions and say they are internal.
- Build the internal size from product plus inner protection plus tolerance.
- Then check the external size against every carrier band you use.
- Place ladder rungs at product clusters, not evenly.
- Board construction changes the external size and can push a band.
Frequently asked questions
Should I specify internal or external dimensions?
Internal, always - and say so explicitly. Board thickness means an external measurement tells you very little about what will fit.
How much tolerance should I allow?
Enough that the packer is not forcing the product in, and not so much that you create a gap you then fill. It depends on the product and the insert; a sample settles it.
What if the right internal size pushes us over a carrier band?
Revisit the specification before accepting it - a thinner insert, a different orientation or a different board construction can often bring the external size back under.
How many carton sizes should we hold?
Fewer than most businesses end up with. Design the ladder against product clusters and carrier bands rather than adding a size whenever something does not fit.
Can you work out the right sizes from our product data?
Yes - send product dimensions and weights for your main lines plus the carriers you use, and we will design the ladder. See bespoke carton manufacture.