A dieline is drawn for a 350 gsm board, the sample looks right, and the production carton jams in the customer’s erecting machine. The weight matched; the thickness did not. Board caliper – thickness – is the quiet specification that decides fit, and it deserves its own answer rather than a footnote on a weight line.
What is caliper? The thickness of a single sheet of board, measured under a standard pressure with a micrometer. It is quoted in microns in Europe and Asia, and in points in North America, where one point equals one thousandth of an inch. The conversion is fixed: one point is 254 microns. A 300 micron board is therefore just under 1.2 points, and quotes from different markets should be normalised to one unit before anyone compares them.
Why do two boards at the same GSM differ in caliper? Because weight and thickness are independent properties. Fibre type, refining and bulk determine how much thickness a given gram count produces – a bulky uncoated board can be visibly thicker than a dense coated board at the same weight. Stiffness depends on thickness cubed more than on weight, which is why two equal-weight boards can feel entirely different in the hand. The stiffness side of this is unpacked in the article on paper weight versus stiffness.
When does caliper matter more than weight? Wherever the board has to fit something. Insert slots and cross-ribbons are cut to a thickness range, and a board a few microns over slides in too tight or cracks at the slot. Machine settings – erecting machines, gluing machines, counter stackers – are set to a caliper window, not a weight. Retail shelf plans may cap total pack thickness. And lamination adds its own microns, which the dieline has to absorb or the fold cracks.
What tolerance is normal on caliper? Board is usually supplied within a stated tolerance of a few microns on nominal thickness, and the tolerance widens slightly across a production run. For ordinary cartons that spread is invisible. For precision fits – a rigid box drawer, a machine-fed carton, a board into a printed sleeve – the tolerance has to be smaller than the clearance designed into the fit, so the fit should be sampled on actual production board, not on a house stock sheet.
How does caliper interact with folding and creasing? Thicker board needs wider crease rules and deeper channels, and a crease set for one caliper can crack a thicker sheet or leave a soft, wandering fold on a thinner one. The mechanics are covered in the article on creasing position, depth and cracking. When a design migrates between weights, the creasing specification should migrate with it rather than being inherited.
How should I check incoming board? A sample from each delivered pallet, measured with a micrometer at several points, catches substitution before it reaches the gluing line. Weight alone will not – a denser board at a heavier grammage can match weight while missing thickness. For orders where fit matters, put the caliper and its tolerance on the purchase order rather than leaving the word gsm to carry the whole specification.
What should I send when specifying a fit-critical pack? The dieline, the machine it must run on with its caliper window, the insert materials and their thicknesses, and the stacking or shelf constraints. The board choice then resolves against all of those at once, and the vocabulary used on quotes is collected in the glossary of packaging quotation terms.
If a machine jam or a tight insert has raised the thickness question, send the dieline and the machine specification. We will tell you which boards at our Wuhan plant sit inside the window and what to trial. Start on the quote request page.
Chengtop produces caixas dobráveis, caixas de presente rígidas and caixas de cartão ondulado at our plant in Wuhan, and we quote board by weight and thickness together on fit-critical work. Message us with the fit that has to hold.
