Mesh is the number of openings in a square inch of a screen or sieve.
It is equal to the square of the number of strands of metal or plastic screening per lineal inch. Ion exchange and filter media are graded by U.S. mesh or screen sizes according to the percent of the medium's particles that will pass through or be retained on certain mesh screens.
Standard U.S. mesh screen #16 equates to a 1.19 millimeter particle diameter; mesh size #40 is 0.42 millimeters. Therefore, media rated as 95 percent -16+40 U.S. mesh would have 95 percent or more of the media particles with sizes between 0.42 and 1.19 mm in diameter.