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The nominal rating of a filter can be expressed by means of this efficiency figure. Given as a percentage, it can be calculated from the beta ratio, or directly from the particle number count:

specific to a particular particle size. It applies over the whole particle size range, down to the absolute cut-off value, at which point the number of emergent particles should be zero, and the efficiency 100%. At any particle size level smaller than the absolute cut-off the efficiency must necessarily be less than 100%.

The classic method of determining filter efficiency is by the bead challenge test, with a rating expressed as a beta ratio, thus a beta ratio 》 75 specifies a filter efficiency of 98.6% or better relative to a particle size of x um.

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