What is the mechanism behind the sandpaper grading system?

There are multiple sandpaper grading systems in existence. The most common one used in the U.S. ranks the grit of sandpaper from coarse to fine, with higher numbers indicating finer grit and lower numbers representing coarser grit. In this system, 16-grit is among the roughest, while 3,000-grit is one of the finest. This system, known as CAMI, was developed by the Coated Abrasives Manufacturers' Institute and is recognized by the American National Standards Institute.

In Europe, the Federation of European Producers of Abrasives (FEPA) has its own sandpaper grading system, which also starts from coarse to fine, but with a 'P' added before the grit number. Up to P240, the CAMI and FEPA systems align closely. For instance, 60 (CAMI) equals P60 (FEPA), 180 (CAMI) matches P180 (FEPA), and 240 (CAMI) corresponds to P240 (FEPA). Beyond that, the systems start to differ. For example, 320-grit (CAMI) is similar to P400, 440-grit (CAMI) equates to P600, 600-grit (CAMI) matches P1200 (a significant difference!), and 800-grit (CAMI) is roughly equal to P1500. It’s essential to know which grading system is used by the manufacturer to avoid using the wrong sandpaper grade and potentially ruining your project.

Besides the U.S. and European systems, there is also the Japanese system. In this system, their 150-grit is equivalent to the 100-grit CAMI, the 1,000-grit matches the 700-grit CAMI, and the 10,000-grit is comparable to the 2,000-grit CAMI. The micron system works differently, where higher numbers indicate coarser grits, in contrast to the other three systems. One micron is one-millionth of a meter, so 535-micron sandpaper corresponds to the rough 36-grit CAMI, while 116-micron sandpaper is similar to the 120-grit CAMI. Sandpaper with a 3-micron measurement equates to 1,500-grit CAMI, and 1-micron sandpaper is like the 2,000-grit CAMI sandpaper.