How to Convert a GSM to an MM
Two common ways of comparing paper quality are to look at its weight, or its thickness. Heavier or thicker paper is usually more durable and more resistant to tears than lighter or thinner paper. If your business goes through a lot of paper, you may soon find that not all paper manufacturers measure paper quality the same way. In fact, paper quality is often measured in three different ways: Basis Weight, GSM
Once you understand what these systems of measurement mean, you can usually approximate how one paper with one type of measurement compares to a similar paper using a different measurement. However, the only way you can get a precise calculation is to use a table provided by the manufacturer. Converting a GSM measurement to an MM measurement of one paper to compare it to another will not give you precise results.
Basis weight is primarily used by U.S. manufacturers. Simply put, it is the weight of 500 sheets of paper – called a ream – at its base size. The base of each sheet of paper is its size before it has been trimmed. For example, if you are buying 24-lb bond paper, which is 8 1/2 by 11 in size, you'll notice that 500 sheets does not weigh 24 pounds. That's because it's weighed at its original size soon after it comes off the rollers, at 17 by 22 inches per sheet.
GSM stands for grams per square meter, which is a metric system of measurement equivalent to pounds per square yard. GSM isn't dependent on the size of the paper before it's been trimmed. Knowing a paper's GSM does not automatically tell you the thickness of the paper. However, if you are comparing two types of the same kind of paper from the same manufacturer, a higher GSM almost always means a thicker paper.
The MM system of measurement, which stands for millimeters, simply measures the thickness of each sheet of paper using calipers. In some cases, the paper's thickness may be measured in points, with each point being 0.001 inch, or 0.0254 millimeters. Of course, knowing the thickness of the paper will give you an idea of its weight compared to other paper of the same kind, but won't give you a precise number on its own.
Most suppliers should be able to give you a table showing how their paper compares in GSM and MM, and sometimes even its basis weight. This is the only way you can accurately compare one paper's measurements to another.
If you can't access these tables, you can approximate the differences yourself, provided you are looking at paper of the same type and quality. Keep in mind, however, that the calculations will not be accurate. This could be due to different recipes being used to manufacture the paper, different types of wood or other materials used to bind the fibers, or even differences in humidity. In short, the weight of the paper doesn't always tell you its thickness, or vice versa.
As an example, each sheet of 60 GSM bond paper from one supplier may be 0.08 mm thick, while 180 GSM bond paper from the same supplier is 0.19 mm thick. If you do the math, you'll see that these numbers don't correspond exactly. The ratio of 60 to 180 GSM is 1:3, but the ratio of 0.08 to 0.19 mm is significantly different at 1:2.375.
It is nearly impossible to compare measurements of different types of paper accurately. The GSM of card stock, for example, can't be converted to MM by comparing it to bond paper.