An Absolute Filtration Rating specifies the particle size, in microns, that a filter or mesh can retain essentially 100% of the time. While nominal ratings measure partial retention, the absolute rating indicates the smallest particle the filter will reliably stop.

In practice, manufacturers often define absolute ratings based on 98–99.999999% retention, depending on testing standards. This rating is useful when precise particle removal is critical, such as in laboratory, industrial, or high-precision filtration applications.