The unwanted particles found in today’s fuels are measured in microns. A micron is a unit of linear measure in the metric system used to measure distance from one point to another. The particles and contaminants that we are trying to filter out are far smaller than can be seen with the naked eye, magnifying glass or even a low powered microscope. A micron rating for a fuel filter is a generalized way of quantifying the ability of the filter’s media to remove contaminants of a particular size.