There are many ways to measure the individual particles that are embedded in abrasives. And because manufacturers differ slightly in their measuring systems, and particle shapes vary so much, it's nearly impossible to offer a standard comparison between grit sizes and micron sizes.
There are, however, some good estimations. Use the information below as a guide to help you determine which abrasive to use. Please note that this table is an approximation and is for comparison only; measurements will vary from one manufacturer to another.
30 microns = approx. 400 grit
15 microns = approx. 600 grit
9 microns = approx. 1200 grit
3 microns = approx. 4000 grit
2 microns = approx. 6000 grit
1 microns = approx. 8000 grit