How to measure a box
Three people can measure the same box and write down three different sets of numbers, all of them correct. This is the single most common way a packaging order goes wrong.
The three readings
| Reading | What it measures | Who uses it |
|---|---|---|
| Inside | The clear space inside the finished box — what actually has to fit. | The corrugated trade quotes internal dimensions by default. If a supplier gives you three numbers with no qualifier, assume this. |
| Crease to crease | The distance between fold lines on the flat blank. | Die-makers and anyone working on the flat file. It is the only one of the three you can measure directly off a dieline. |
| Outside | The external footprint of the assembled box. | Anyone fitting boxes into something else — a shelf, a pallet, a carrier's size band, an outer case. |
How far apart they are
One board thickness per step. Take a box in 4 mm C flute:
- crease-to-crease is about one caliper larger than inside — 4 mm on each dimension
- outside is about one caliper larger again — so outside is roughly 8 mm larger than inside on every dimension
On a 300 × 200 × 150 mm inside box that is a 316 × 216 × 158 mm outside box. Get the reading wrong on a carrier size band or a pallet layout and that is the difference between fitting and not.
The generator applies exactly this rule. Enter dimensions in whichever mode matches what you have and it shows all three readings as you type, so you can check against whatever your supplier asked for.
The honest limit of that arithmetic. One caliper per dimension per step is the standard first-order approximation, and it is right for a simple tube like an 0201. It is only approximately right for constructions that double the board up — a rolled-end mailer, a double-wall tray, a liner inside a case. Those lose more internal space than a single caliper.
The allowance is exposed as an adjustable factor rather than hidden, so you can tune it. It is still an approximation. Cut and fold a sample.
Which order do the numbers go in
Length, then width, then height. Look at the open face of the box: length is the longer side of that opening, width is the shorter side, and height is the depth from the opening down to the base.
The FEFCO 12th Edition writes them as L, W and H. Older material and some suppliers write B for breadth where you would expect W. It is the same measurement under an earlier notation, not a different one.
Some styles carry a fourth figure. An overlap slotted container is quoted as something like
355 × 205 × 120/40 mm, where 40 is the overlap on the flaps.
Measuring an existing box
- Open it out flat if you can. Measure crease to crease with a steel rule, from the centre of one fold line to the centre of the next.
- If you cannot flatten it, measure the inside with the box squared up. Push the rule right into a corner rather than reading across the middle of a bowed panel.
- Measure the board with callipers on a cut edge, away from a crease. Creased board is compressed and reads thin.
- Enter the reading you took, in the matching mode, and check the other two against the box in your hands.
Measuring for a product you have not boxed yet
Measure the product at its largest point, including any bulge, seam or label. Then add clearance — somewhere between 2 and 5 mm per dimension for a rigid product is a reasonable starting point, more for anything soft, and more again if a person has to get their fingers in to lift it out.
That clearance is yours to decide. The generator's allowance covers the board only; it knows nothing about your product. Enter the inside dimension you want, clearance already included.
Where it usually goes wrong
- Quoting outside dimensions to a converter. They will normally read them as internal, and you get a box one board thickness small in every direction. Say which you mean.
- Measuring a bowed panel across the middle. A loaded box bulges. Measure at the corners.
- Using the wrong caliper. A dieline built at 1.5 mm and cut in 4 mm board will not close. If a box binds, check the board setting before you touch anything else.
- Forgetting the height convention. Height is measured from the opening, not from whichever face happens to be sitting on the bench.
Related
- Corrugated flutes and board calipers — the thickness figures behind the arithmetic above.
- FEFCO codes explained — picking the construction before you size it.
- FAQ — file formats, board choice and why a box might not fold.