What is caliper in board?

Caliper is the more useful measurement in packaging because thickness is what the machinery interacts with. Crease width is set from it. The gap the folder-gluer runs at is set from it. And the internal dimensions of a finished carton differ from the flat drawing by an amount derived from it, because the board takes up space at every corner.

That last point is where downloaded templates go wrong. A dieline drawn for 350 micron board and used at 500 micron produces a box whose creases are too narrow, whose corners bind, and whose internal dimensions are smaller than intended — sometimes enough that the product no longer fits. The drawing looks correct; the box is not.

On rigid boxes caliper matters even more, because the wrap has to travel around the thickness of the greyboard on every edge and the wrap sizes are calculated from it.

The practical implication for a buyer is simple: specify the board before the dieline is finalised, and if you change board later, expect the dieline to be redrawn. It is not a formality, and on a job where the die is already cut it is a new tool.

One note on units. Caliper is quoted in microns almost everywhere except North America, where points are still common. A point is a thousandth of an inch, so 24 point is roughly 610 microns — useful to know when reading a specification or a board swatch written for a different market.