Filter media is sometimes rated in U.S. Mesh instead of microns. This gauge standard is the number of openings per linear inch using standard wire diameters. Wire mesh measures the number of wires (or threads) per linear inch, not the size of the holes between them. As the number of wires per inch goes up, the size of the holes (in microns) goes down.
The chart below roughly converts between U.S. Mesh, microns, and fractions of an inch. There is no simple formula to convert between microns and wire mesh because it would have to account for changing wire diameter.