Here are some important links that you might find helpful.
The term ‘Mesh’ is used to describe the size of a particle. This indicates the mean or average size of the range of particles in that grade. U.S. Mesh Size is defined as the number of openings in one square inch of a screen (e.g. No. 35 Mesh will have 35 openings per square inch). The chart below shows the approximate size in inches, millimetres and microns for numerous mesh sizes.