Below you can download a table showing how the various scales by which the grit of an abrasive is measured relate to each other.
Here are the main scales:
FEPA – Fédération Européenne des Fabricants de Produits Abrasifs, European Federation of Abrasives Manufacturers. FEPA distinguishes between the abrasive grain used for flexible abrasives, i.e. cloths and abrasive papers (FEPA P scale) and that used for rigid abrasives, i.e. wheels with a hole or shank (FEPA F). We are therefore interested in the ‘P’ scale. The grits are defined by the number of lines per inch (25.40 mm) on the sieves used to separate them from each other. A P150 grit therefore has to pass through a sieve with 150 lines/inch, but not through a sieve with 180 lines/inch, through which P180 passes. And so on.
MICRON – A micron, also referred to by the Greek letter μ is one thousandth of a millimetre (0.001 mm). It is the scale used to measure micro grits not included in the FEPA scales, and, most interestingly for us, is used to measure the grits of diamond and CBN abrasives.
JIS – Japanese Industrial Standard. This is the scale issued by the Japanese Standards Association. Rarely used: Even the Japanese tend more and more to use the FEPA scale.
ANSI – American National Standards Institute. This scale is also rarely used in the field of flexible abrasives.