Grey board: what caliper you actually get
Grey board is bought by stiffness, and stiffness comes from thickness. But two boards at the same grammage can differ by more than a tenth of a millimetre — which is the difference between a rigid box that closes and one that does not.
The number nobody puts on the order
Orders arrive specifying grammage. Grammage tells you what the board weighs, not how thick it is. Thickness at a given weight is bulk, and bulk is a property of how the board was made — what furnish went in, how hard it was pressed, how many plies.
The table below is measured caliper from the mill's own data sheet, not a calculation. Read across a row: that is the same weight of board arriving at three different thicknesses.
| Grammage | Pure pulp | Deluxe double | Double-sided |
|---|---|---|---|
| 250 g/m² | not run | 410–425 µm | 415–435 µm |
| 300 g/m² | 460–480 µm | 510–525 µm | 510–530 µm |
| 350 g/m² | 540–560 µm | 600–620 µm | 610–630 µm |
| 400 g/m² | 620–640 µm | 700–720 µm | 710–730 µm |
| 450 g/m² | not run | 785–810 µm | 800–820 µm |
Caliper ranges as published by the mill (XSD grey board data sheet). Confirm against the data sheet for the run you are buying.
How to read it
Pure pulp board is the densest of the three: at every weight it runs thinnest. That is the point of it — a hard, close-formed board that stays flat, cuts cleanly and takes a surface. It is only made at the heavier end, from 300 g/m² up.
The deluxe double and double-sided tiers are bulkier. At the same weight they arrive thicker, which means more stiffness per tonne bought. Where the board is laminated or covered anyway and the finished surface is not the board itself, that is the economical answer.
The widest gap in the table is at 400 g/m²: pure pulp board runs 620–640 µm while double-sided runs 710–730 µm. The ranges do not overlap — 70–110 µm apart, about 14% more thickness for the same tonnage.
Where a row is blank, that tier is simply not run at that weight. It is not an omission — asking for it produces a special run, not a stock item.
What to put on the enquiry
Give the caliper you need and the tolerance you can live with, then the grammage as a secondary constraint. If the box is die-cut, say so — the cutting rule is set to thickness, and a board that arrives 40 µm off will show it on every crease.
Sheet size, grain direction and moisture content are worth stating at the same time. All three are confirmed with the quotation.
Questions
How thick is 400 gsm grey board?
It depends on the tier. On the mill data we work from, at 400 g/m² pure pulp runs 620–640 µm, deluxe double runs 700–720 µm, double-sided runs 710–730 µm — the same weight arriving anywhere from 620 to 730 µm. Confirm against the data sheet for the run you are buying.
Why does the same grammage come in different thicknesses?
Because thickness at a fixed weight is bulk, and bulk depends on furnish and pressing. A bulkier board holds more air, so it is thicker and stiffer for the same mass.
Can you supply grey board outside these weights?
The weights listed are what these tiers are run at. Anything else depends on the mill and the run and is confirmed with the quotation — we do not publish a spec sheet that the mill has not agreed to.
Related grades
Grey board