GSM vs PT Unit System: What Is the Difference?
The PT stands for point, while GSM is an acronym for grams per square meter. Both are used to measure the thickness of stock (paperboard/card/other packaging material) and typically used to determine material stock for folding carton.
The Point System (PT)
Essentially, a point is equivalent to 0.001 inch. 10 points would mean a stock thickness of 0.01 inch.
This is the metric equivalent of the point system, relying on centimeters and meters (1 meter = 100 cm) instead of inches.
An inch corresponds to 2.54 centimeters or 0.0254 meters, but then again, it isn’t easy converting points into GSM as the latter also factors in weight, in addition to area (length and breadth; instead of the height/thickness measured by points).
However, a lot of packaging materials are standard, so here is a rough table to help you compare the two systems side by side.
|Thickness / Packaging Material||Weight|
|13 point Pulp||200 gsm|
|14 point Uncoated||270 gsm|
|16 point C2S / Solid Bleached Sulfate (SBS)*||350 gsm|
|18 point C2S||400 gsm|
|24 point Uncoated||405 gsm|
|24 point C2S / Solid Bleached Sulfate (SBS)*||460 gsm|
|40 point Cotton||600 gsm|
*Solid Bleached Sulfate (SBS), also called Solid Bleached Board, when coated on both sides is known as C2S. It is called C1S if the coating is on only one side.
As you can see, more thickness does not simply translate into more weight. A 17-point uncoated surface packaging material weighs 350 gsm, but the thicker 18-point brown Kraft weighs only 315 gsm.
Because much of the world relies on the metric system, gsm is more in use. Here are some examples to help you understand paper better.
35-55 gsm: This is the paper that most newspapers are printed on.
90-100 gsm: This is normal printer/copier paper. Something more premium, like bond paper, would be in the region of 110-120 gsm.
120-140 gsm: Flyers and posters tend to be printed on this paper, because it is stronger and can resist some wear and tear.
210-300 gsm: If you have seen and experienced glossy brochures, you know what this paper feels like.
350-400 gsm: Used for mainly packaging & business cards
But apart from paper, there are also other packaging materials that are used, such as corrugated board (there are various types like single face, single wall, double wall and triple wall – and all of them have different gsm).
The Implications for Packaging
Points or gsm alone cannot be the deciding factor when choosing a packaging material. These are measures of thickness and weight respectively, and although greater values indicate higher strength, there is the also the actual material to be considered.
Some materials have a smooth surface, making printing on them easy, while others have a coarser surface. The quality of the print on such surfaces declines with lesser smoothness, but there is the primary function of packaging which comes first – to protect the contents inside.
For the right packaging material that serves both objectives – protection as well as better printing on the surface – and cost, you would do well to talk to an experienced printer. See (and feel) some samples before making your decision.