Specify a cardboard box by internal dimensions, board grade, flute and style - in that order. Internal dimensions are taken from the product plus its protection, because an external measurement includes the board and will leave you short. Everything else follows from weight and how the box is stacked.
The four decisions, in the order they matter
Nearly every carton problem traces back to one of four decisions being made badly or not at all. They are not equally important, and they are not independent - but taken in order they produce a specification rather than a guess.
- Internal dimensions, measured from the product plus whatever protection sits around it. Specify internally; an external figure includes the board thickness and leaves the product short.
- Board grade, driven by weight and by how the carton will be stacked. A heavy item carried alone and the same weight in a palletised stack need different things.
- Flute profile, which decides cushioning, printability and how the board behaves when it is folded.
- Box style, because a regular slotted case and a die-cut box suit very different pack rates and very different products.
If you take one thing from this: the gap between the product and the box is paid for twice - once in the void fill that occupies it, and again in the volume a carrier charges to move it.
Getting the dimensions right
Measure the product at its largest point in each axis, add the protection you intend to use, and then add a working tolerance - enough that the product goes in without being forced, and not so much that it moves. A product that can move inside its box is being damaged by its own packaging.
Order the dimensions consistently as length x width x depth, internal, and say so on every drawing and every enquiry. A surprising proportion of carton errors are nothing more than two parties reading the same three numbers in a different order.
| Element | Why it changes the internal size |
|---|---|
| The product at its widest point | Not its nominal or average size |
| Protective material | Foam, fill or interleave all take space |
| Multiples | If more than one unit ships, the configuration matters |
| Working tolerance | Enough to pack quickly, not enough to rattle |
| Documentation | A packing note or label inside takes room |
| Closure method | Tape, glue or interlocking flaps behave differently |
What the dimensions have to account for.
Board grade, flute and style
Board is described by its liners and its fluting, and the shorthand hides the two things that actually matter in use: burst strength, which determines whether a corner survives a point load, and edge crush, which determines whether a stacked carton holds up the ones above it. The full range of grades sits under packaging supplies, and the made-to-size route under bespoke carton manufacture.
| Flute | Character | Typically used for |
|---|---|---|
| A | Thickest, most cushioning | Fragile goods, low stacking |
| B | Thin, good crush resistance | Canned and tinned goods, retail outers |
| C | The general-purpose compromise | Most shipping cartons |
| E | Fine, prints well | Retail and presentation cartons |
| BC double wall | B and C combined | Heavy or palletised despatch |
| EB double wall | Fine outer, strong inner | Printed heavy retail packs |
Flute profiles and what each suits.
Style then follows the pack rate. A regular slotted case is cheap, flat and fast to assemble by hand. A die-cut or crash-lock base saves seconds per pack and suits a bench under pressure. A fitted or telescopic box costs more and earns it on anything where presentation matters. See packaging supplies for the ranges.
Where carton specifications usually go wrong
- Specifying external dimensions - the single most common error, and it leaves the product short every time.
- Defaulting everything to double wall, which feels safe and is simply more expensive on every parcel that never needed it.
- Sizing by eye for dense goods, which produces cases that are structurally fine and too heavy to lift.
- Ignoring the stack, because a carton that survives alone can fail under three of its own kind.
- Keeping a size after the product changed, which is how a range silently stops fitting.
- Treating fill as a fix, when heavy fill use is a symptom that the carton is wrong - see Void Fill Compared: Paper, Air and Loose Fill.
When a standard size stops being the right answer
A standard carton is the right answer far more often than bespoke suppliers suggest, and the wrong answer far more often than businesses realise. The test is simple: if a standard size leaves a gap you fill on every single despatch, you are paying for that gap in board, in fill, in volumetric carriage and sometimes in damage - on every order, for the life of the product.
At that point a made-to-size carton frequently costs less delivered than the standard box plus everything it needs. The obstacle is usually the run quantity rather than the unit price, and that is a storage problem with a straightforward answer - see bespoke carton manufacture and packaging stockholding.
The short version
- Specify internal dimensions, taken from the product plus its protection.
- Board grade follows weight and stacking, not the trade the product belongs to.
- Flute decides cushioning and printability; style decides pack speed.
- Heavy void fill use is a symptom of a wrong carton, not a protection requirement.
- Bespoke earns its place when a standard size leaves a gap on every despatch.
Frequently asked questions
Should I give internal or external box dimensions?
Internal, always, and say so explicitly. External dimensions include the board thickness, so a box ordered to external measurements is smaller inside than intended - which is the most common carton specification error there is.
What board grade do I need?
It depends on the weight and on how the carton is stacked rather than on what is inside it. Single wall handles most parcel work; double wall earns its place when cartons are palletised, stacked or travelling long distances.
What does the flute letter mean?
It describes the size and shape of the corrugations between the liners, which sets cushioning, crush resistance and how well the board takes print. C flute is the general-purpose choice; E flute prints finely; BC double wall is for heavy or stacked work.
How much space should I leave around the product?
Enough to pack it quickly without forcing it, and not enough for it to move. A product that rattles is being damaged by its own packaging, and filling a large gap with void fill treats the symptom rather than the cause.
Is a made-to-size box worth it?
When a standard size leaves a gap you fill on every despatch, usually yes - the saving shows up in fill, carrier band and damage rather than in the box price. See Bespoke Packaging: What the Tooling Actually Buys You.