The relationship between mesh and micron is often very confusing. Simply, mesh and micron are ways to identify the particle size of a specific grit or particle.
Mesh is a count of the number of openings in one square inch of screen. A 36 mesh screen indicates there are 36 openings; 120 mesh, 120 openings, etc. As the number of openings increases and the number gets larger, the size of the grit decreases. Sieve size and mesh size are often used interchangeably.
Micron is exactly 1/1000 mm (one millionth of a meter) or 1/25,000 of an inch. Very fine grit sizes are often noted in microns (instead of mesh) since the size of the openings on a screen becomes less important relavative to diameter of the screen wire. Smaller than 325 mesh is more accurately designated by micron size.
A formulation was developed by Don Kirk to perform a rough conversion mesh to micron or vice versa. Kirk explains that the following formulation has a conversion error of 7.6% or less over the 45-400 mesh range. This is a rough but convenient calculation when conversion charts are not available.
Microns = 15,000 / Mesh (or Mesh = 15,000 / Microns)
Inches = 0.6 / Mesh (or Mesh = 0.6 / Inches)