Materials & Board · Quick guide
GSM and board thickness explained: GSM, microns and points
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GSM tells you how heavy a board is, and calliper, measured in microns, tells you how thick it is. The two are linked by the board's density, so two boards with the same GSM can be different thicknesses and have different stiffness. To specify a carton properly you need the GSM and the board type; to compare it with a North American quotation you need to convert points to microns.
This guide explains each measurement, how they relate and which figure to use when. For the boards themselves, see our pages on folding boxboard and SBS, greyboard and corrugated board.
GSM: the weight of the board
GSM stands for grams per square metre. Cut a sheet of board exactly one metre by one metre, weigh it, and the figure in grams is its GSM. It is the standard way to specify paper and carton board in the UK and Europe, sometimes written g/m² or called grammage.
There is no hard line between paper and board, but material above roughly 200–250 gsm is usually called board. Packaging uses a wide spread of weights, and each part of a box tends to sit in its own range.
| Material or part | Typical GSM | Notes |
|---|---|---|
| Wrap paper for rigid boxes | Commonly 120–157 gsm | Light enough to turn over edges and corners cleanly |
| Speciality wrapping papers | Commonly 100–170 gsm | Textured, metallic and coloured papers |
| Corrugated liners | Commonly 115–200 gsm | Each liner is specified separately |
| Small cartons, sleeves, inserts | Typically 250–300 gsm | Folds neatly on small panels |
| Everyday folding cartons | Typically 300–350 gsm | Cosmetics, supplements, food cartons |
| Larger or heavier cartons | Typically 350–400 gsm | Stiffer panels and load-bearing bases |
| Greyboard for rigid boxes | Around 900–2,400 gsm | Specified by thickness, typically 1.5–3 mm |
Figures are typical ranges and vary by supplier and mill.
Calliper and microns: the thickness
Calliper is the thickness of a single sheet. In the UK it is quoted in microns (µm), where 1,000 microns is 1 mm, and it is measured with a precision gauge under a set pressure. A typical carton board sits somewhere between about 300 and 650 microns.
Greyboard is thick enough to be quoted in millimetres, typically 1.5–3 mm. Corrugated is described by its flute rather than a calliper figure, because the flute sets the thickness: E flute is about 1.5 mm, B flute about 3 mm and C flute about 4 mm. Our E vs B vs C flute comparison explains the differences.
Calliper matters for fit. The thickness of the board affects how a carton folds, how tightly a tuck flap closes, and how much space is left inside once the walls are formed.
Density and bulk: why the same GSM can mean different thickness
GSM and calliper are connected by density. A dense board packs its weight into a thinner sheet; a bulky board spreads the same weight over a thicker one. Papermakers often describe this as bulk, which is simply the calliper in microns divided by the GSM.
| Board at 300 gsm | Typical calliper | Approximate bulk | Why |
|---|---|---|---|
| Folding boxboard (FBB) | About 430–480 µm | About 1.4–1.6 | Bulky mechanical-pulp core |
| Solid bleached board (SBS) | About 370–420 µm | About 1.2–1.4 | Bleached chemical pulp throughout |
| Recycled carton board | Varies widely by mill | Often lower than FBB | Shorter, denser recycled fibres |
These figures are indicative; every mill publishes its own data.
Density matters because stiffness rises very steeply with thickness. Roughly speaking, doubling the thickness of a sheet makes it around eight times stiffer to bend, which is why a bulky FBB is stiffer than an SBS of the same weight. Mills publish stiffness directly, usually as bending resistance in millinewtons (mN), and that is the most reliable way to compare how two boards will perform as a carton panel.
Converting North American points to microns
North American suppliers quote board calliper in points. One point is a thousandth of an inch, which is exactly 25.4 microns. So the conversion from points to microns is simple: multiply by 25.4.
Converting points to GSM is not simple, because it depends on density. The table below gives indicative GSM ranges, from a denser SBS at the lower end to a bulkier FBB at the higher thickness for the same weight. Use it as a guide for comparison, not a specification.
| North American points | Calliper in microns | Calliper in mm | Indicative GSM range |
|---|---|---|---|
| 12 | 305 µm | 0.30 mm | About 200–240 gsm |
| 14 | 356 µm | 0.36 mm | About 235–280 gsm |
| 16 | 406 µm | 0.41 mm | About 270–320 gsm |
| 18 | 457 µm | 0.46 mm | About 300–360 gsm |
| 20 | 508 µm | 0.51 mm | About 335–400 gsm |
| 24 | 610 µm | 0.61 mm | About 400–480 gsm |
| 28 | 711 µm | 0.71 mm | About 470–560 gsm |
Rigid board in North America is also quoted in points, often in the region of 50 to 90 points. That converts to roughly 1.3–2.3 mm; in the UK the same board is specified in millimetres, typically 1.5–3 mm.
Which figure should you specify?
Use the figure that fits the material, and always give it with the board type.
- Folding cartons. GSM plus board type, such as "350 gsm FBB, cream back". Calliper is useful as a check, especially when comparing boards.
- Rigid boxes. Greyboard thickness in millimetres, plus the wrap paper and any lamination.
- Corrugated. Flute and liners, such as "E flute, white outer liner, kraft inner", with the liner weights if you know them.
- Papers and wraps. GSM plus the paper type and finish.
You do not need to work any of this out before asking for a quotation. Tell us the product, its dimensions and weight, and how it is sold or shipped, and we will recommend the grade. Our guide to comparing packaging quotes shows how to check that two quotations describe the same board.
Common mistakes
- Treating GSM as thickness. GSM is weight; thickness is calliper. The same GSM can mean quite different thicknesses.
- Comparing GSM across board types. A 300 gsm recycled board and a 300 gsm FBB will not feel or perform the same.
- Converting points straight to GSM. Points convert to microns exactly, but to GSM only approximately; match on calliper and board type instead.
- Assuming heavier is always better. Beyond the weight the product needs, extra GSM adds cost and shipping weight without adding useful performance.
- Forgetting the board uses up internal space. Thicker board takes room inside the box; see internal vs external dimensions.
- Quoting a box size without the board. Two quotations for the same box in different grades are not comparable; ask for the grade in writing. Our packaging materials guide explains how each material is usually specified.