A nominal micron rating describes the approximate size of the smallest particle a filter or mesh can retain, expressed in microns. Specifically, it represents the particle diameter that the filter captures with 60–90% efficiency, rather than guaranteeing complete removal.

For example: A 10-micron nominal rating can capture most particles around 10 microns, but some may pass through.

This rating helps compare mesh or filter performance for filtration, screening, or separation applications.