Micron ratings can be very misleading since there is not a single standard test that is used by all manufacturers. This means that ratings can have a wide variation, depending on the test method used. The estimated mean flow pore (MFP) size for the Reef Diaper is 65 microns. Estimated MFP is calculated using basis weight, permeability and thickness. The question is further complicated by the fact that micron rating is constantly decreasing as the filter becomes dirty and the pore size is reduced. The best method for evaluating a filter is to look at the results in your own aquarium. Based on our tests and customer experience, the Reef Diaper has the performance and capacity comparable to a 200 micron felt sock.