I'm obviously a bit thick, ignorant or both, I recently bought some silicon carbide powder F800 thinking it was similar in grade to P800 abrasive paper; it's not, its more like P3000! (about 8 microns, P800 paper being 22 microns).
F and P numbers are part of the FEPA European system: P for abrasive papers, F for abrasive powders; U.S. ANCI system numbers have no letter prefix; Japanese JIS system may use a J prefix.
In the past I've used terms like 600g for 600 grit - this seems to be nonsense.
Here is the best comparison chart (Russian) I could find which includes FEPA, ANSI, JIS, MICRONS, and other water stone types like Shapton etc. hope this is of help to other site users.
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