I suspect that the wording of this question is not quite technically correct. I am assuming that the person asking this question is enquiring about the unit of measurement ('mesh') that is used for measuring the sizes of beads and meshes (eg wire meshes). If this is the case, within the United States, the mesh size, is equal to the number of openings per one inch. U.S. Mesh --> Inches --> Microns ------ 3 --> 0.2650 --> 6730 4 --> 0.1870 --> 4760 5 --> 0.1570 --> 4000 ... 10 --> 0.0787 --> 2000 25 --> 0.00280 --> 707 ... 100 --> 0.0059 --> 149