Filter media is sometimes rated in U.S mesh and also in microns which makes life confusing for us all. It really comes down to where the filter media is made and which unit of measure the manufacturer chooses. 

U.S mesh 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. 

This chart below is a rough conversion between U.S Mesh and Microns as there is no simple formula to convert between microns and mesh because it can not account for the wire diameter.

U.S. Mesh
Microns
10
2000
12
1680
14
1410
16
1190
18
1000
20
840
25
710
30
590
35
500
40
420
45
350
50
297
60
250
70
210
80
177
100
149
120
125
140
105
170
88
200
74
230
62
270
53
325
44
400
37
550
25
800
15
1250
10

You can now use this chart to easily understand what size filter you require and convert from U.S. Mesh to Microns.


Spraying Painting gun filters