Wire Cloth Specification TablesOpenings 1/4 inch and over are referred to as Space Cloth (1/8"-4" openings) Mesh is designated by the opening between wires.
Openings less than 1/4 inch are referred to as Mesh cloth (2-635 mesh) Mesh is designated by the number of wires per lineal inch.
Mesh can also be designated by the center to center distance from one wire to the next Center to Center cloth
For very tight weave see Filter Cloth
Mesh Calculator can be used to calculate open area percentages.