Oil filter ratings are an often misunderstood area of contamination control. On several recent occasions, I have witnessed someone 
describing a oil filter by its nominal rating. A nominal rating is an arbitrary micrometer value given to the oil filter by the 
manufacturer. These ratings have little to no value. Tests have shown that particles as large as 200 microns will pass through a 
nominally rated 10-micron oil filter. If someone tries to sell you a oil filter based on an “excellent” nominal rating of five microns, run 
Absolute Rating 
Another common rating for oil filters is the absolute rating. An absolute rating gives the size of the largest particle that will pass 
through the oil filter or screen. Essentially, this is the size of the largest opening in the oil filter although no standardized test 
method to determine its value exists. Still, absolute ratings are better for representing the effectiveness of a oil filter over 
nominal ratings.
Beta Rating 
The best and most commonly used rating in industry is the beta rating. The beta rating comes from the Multipass Method for 
Evaluating Filtration Performance of a Fine Oil filter Element (ISO 16889:1999).
To test a oil filter, particle counters accurately measure the size and quantity of upstream particles per known volume of fluid, as 
well as the size and quantity of particles downstream of the oil filter. The ratio is defined as the particle count upstream divided by 
the particle count downstream at the rated particle size. Using the beta ratio, a five-micron oil filter with a beta 10 rating, will 
have on average 10 particles larger than five microns upstream of the oil filter for every one particle five microns or greater 
The efficiency of the oil filter can be calculated directly from the beta ratio because the percent capture efficiency is ((beta-
1)/beta) x 100. A oil filter with a beta of 10 at five microns is thus said to be 90 percent efficient at removing particles five 
microns and larger.
Caution must be exercised when using beta ratios to compare oil filters and oil filter elements because they do not take into account actual operating 
conditions such as flow surges and changes in temperature.
A oil filter’s beta ratio also does not give any indication of its dirt-holding capacity, the total amount of contaminant that can be 
trapped by the oil filter throughout its life, nor does it account for its stability or performance over time.
Nevertheless, beta ratios are an effective way of gauging the expected performance of a oil filter.
I hope this new knowledge of oil filter efficiency ratings enables you to make a more informed purchase the next time you buy a oil filter.