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A nominal rating is an arbitrary value for the performance of a filter, determined by the filter manufacturer, and expressed in terms of percentage retention of a specified contaminant (usually spherical glass beads) of a given size. It also represents a nominal efficiency figure for the filter. Figures typically quoted are at the level of 90, 95 or 98% retention of the specified particle size. Many filter manufacturers use such tests, but the lack of uniformity and reproducibility has caused this measure to fall into disfavour.

The variations can be quite large. For example, a felt element with a nominal rating given as 30 um may well pass 20–40% of particles of this size. At the same time, it may well retain a significant proportion of much smaller particles. This retention of undersize will, of course, always occur, the actual amount depending upon the design of the element.

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