Large sieve opening (1 to 1/4 inch) have been designated a sieve "mesh" size that corresponds to the size of the opening in inches. Sieve "mesh" sizes of 31/2 and higher are designated by the number of openings per linear inch in the sieve.
Special conventions apply to particle sizing by mesh designation. A "+" before the sieve mesh indicates that all particles are too large to pass through the sieve; a "–" denotes complete passage of material through the sieve. For example, the particle size of a material may be described as –4 +40 mesh, meaning the material goes through a 4-mesh sieve (particles are smaller than 4.76 mm) but remain on the 40-mesh sieve (particles are larger than 0.420 mm).
Note: To convert millimeters to microns, move decimal point three places to the right. Example: 0.125 millimeters converts to 125 microns.