In the paper industry, the abbreviation 'gsm' refers to "grams per square meter." In other words, a square meter of a specific type of paper will weigh a certain number of grams. This designation tells paper consumers how dense the paper is, which generally indicates quality. This connotation differs from the way paper is designated in the United States, which is measured in pounds per 500 sheets. A conversion chart must be used to compare pounds to gsm.
Definition of gsm
According to the University of Cambridge, gsm is "an abbreviation for 'grams per square meter,' a measure of density. It is often used to give the density of paper." The abbreviation is not capitalised. It can also be written g/m2, which is the notation preferred by the International System of Units (SI). In the paper industry of Europe and other regions that employ the metric system, a sheet of paper of size A0 is exactly one square meter. The weight of an AO sheet of any kind of paper is the gsm for that kind.
Although the quality, and therefore the gsm, of the same type of paper can vary among manufacturers, there are some generally accepted standard gsm designations for common paper types.
For example, paper used for the pages in a standard telephone book is usually about 60 gsm. Regular copy or laser printer paper is around 80 gsm. Inkjet paper can vary from 120-310 gsm. Photo prints are about 250 gsm.
What This Means in the USA
In the United States, paper is measured in quite another fashion and designated in pounds. A sample of 500 sheets of the standard size for a particular paper is weighed, and that weight in pounds is assigned to the paper.
Confusion arises among consumers because the standard sheet size varies for different uses of paper. For instance, the sheet size for copy paper is different from the sheet size for book pages. Both of these are different from the sheet size for cover paper.
An Example Comparison
As an example, consider the paper used for copy machines or laser printers. The standard sheet size of this paper is 17 inches by 22 inches. Therefore, 500 sheets of a common density of this paper can weigh 9.07kg. and is referred to as 9.07kg. paper.
On the other hand, the standard sheet size for offset book paper is 25 inches by 38 inches. 500 sheets of a similar quality of this paper weighs 22.7kg. and is referred to as 22.7kg. paper.
That same paper is designated at about 75 gsm.
A conversion chart is the most expedient way to compare the designations of paper in pounds and gsm. Each printer and paper manufacturer has its own variation on the paper types, which can mean that the weights will differ from other sources. When doing business with any paper firm, make sure to use their own chart for printing purposes. However, a general conversion chart is useful when comparison shopping. Charts with general guidelines for conversion can be found online.