We all know about sandpaper grit. The lower the grit, the more coarse the sandpaper is, while the higher the grit, the less material it removes (and the smoother you can make your surface). You may have noticed that sometimes the grit has a “P” preceding the number. What does that mean?

It turns out sandpaper grit is measured in two different scales. As with most types of measurements, you have the US way and the European way. The US scale uses the bare numbers and is known as CAMI (Coated Abrasive Manufacturers Institute). The European scale is the “P” grade, and is known as FEPA (Federation of European Producers of Abrasives).

Both scales are based on the diameter of the average particle size in micrometer (µm), also know as a micron (one millimeter equals one thousand microns). Below is a chart comparing the two.

You can see that the two different grit scales are fairly comparable at the coarser end of things.  In fact, 180 and P180 are exactly the same. But once they start getting finer than about 240, the two scales really start to diverge; 600 and P1200 are nearly identical.

Most sandpaper sold in the US uses CAMI, but you can find some brands with the “P” scale; Klingspor is one that comes to mind. The two scales can get tricky at the finer ends of things. If you read a tutorial that says to sand something at 800 grit, but you grab a sheet of P800 sandpaper, you’ll be using something far more coarse than what was intended.

But now you know.