Actually, there is more than one sandpaper numbering system. The most common system used in the U.S. measures the grits of sandpaper in an ascending order, with the finest grit getting the highest number and coarser grits having lower numbers. In this system, 16-grit is one of the coarsest grades of sandpaper, while 3,000-grit is one of the finest. This system is known as CAMI, as it was developed by the Coated Abrasives Manufacturers' Institute, and was approved by the American National Standards Institute.
In Europe, the Federation of European Producers of Abrasives (FEPA) has its own system for grading grits of sandpaper. Again, they start from coarse to fine, with the addition of the letter P before the grit number. Until around P240, the two systems are approximately in sync, with 60 (CAMI) equaling P60 (FEPA), 180 (CAMI) similar to P180 (FEPA) and 240 (CAMI) equaling around P240 (FEPA), but then the two begin to diverge. The 320-grit (CAMI) is approximately equal to P400, 440-grit (CAMI) is P600, 600-grit (CAMI) is P1200 (big difference!) and 800-grit (CAMI) is the equivalent of P1500. It's important to keep in mind which system is being used by your sandpaper manufacturer and the instructions you have for how to sand something. Otherwise, you might end up using the wrong grade of sandpaper on your project and ruining it.
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In addition to the U.S. and European system, there is the Japanese system. Their 150-grit is equivalent to the 100-grit CAMI, the 1,000-grit measures up to the 700-grit CAMI and the 10,000-grit is the same as the 2,000-grit CAMI. In the micron system, the higher the number, the coarser the grit, unlike the other three systems mentioned. One micron is a millionth of a meter, so 535-micron sandpaper corresponds to the coarse 36-grit CAMI sandpaper, and 116-micron sandpaper is similar to 120-grit CAMI sandpaper. The 3-micron sandpaper is equivalent to 1,500-grit CAMI sandpaper, and 1-micron sandpaper is like 2,000-grit CAMI sandpaper.
