Sandpapers are labeled to show their coarseness according to different standards. [The CAMI system is used in e.g. the USA, and the FEPA system in Europe][1]. I am interested in the FEPA system. It labels sandpaper with a number such as 220, 360, 500, 1000 etc. where higher numbers indicate finer coarsenesses. Exactly how does this number tell coarseness? I am interested in the FEPA number definition only.

I found [this website][4]. It states that:

> [...] by the FEPA standard for macrogrits grade F180, no more than 3% by mass of the grit can have a particle size larger than 90 microns, and at least 94% must be larger than 53 microns. In F220 (a “microgrit”), no more than 3% can be larger than 75 microns, at least 50% must be in the range 50.0 to 56.0, and at least 94% must be larger than 45 microns. See the Standards section below for detailed information.

So:

- **180**: Grain sizes mainly within $[53;90]\;\mathrm{\mu m}$
- **220**: Grain sizes mainly within $[45;75]\;\mathrm{\mu m}$

This indicates that the FEPA number (I don't know why they call the numbers `F`; they are usually [denoted][1] with a `P`) indicates a range of *grain sizes*.

Do anyone know more and can confirm or better explain the definition and for example know where to find an overview of more FEPA numbers?


Many websites, [such as this][1], explain how to understand the standard but not how the number relates to a physical value. The [Wikipedia article][2] is not of much help as it doesn't dive any deeper into the definition details than other websites I can find. And the [FEPA website][3] is of no help either since it seems to require payment for the information.

Thank you for you help.


---

## Update ##

I have been looking into sandpaper types.

 - FEPA P220 or CAMI 220 have a grain size of $68\;\mathrm{\mu m}$
 - FEPA P220 or CAMI 220 have a grain size of $68\;\mathrm{\mu m}$

  [1]: http://www.doityourself.com/stry/a-complete-guide-to-sandpaper-grit-classification
  [2]: https://en.wikipedia.org/wiki/Sandpaper
  [3]: https://www.fepa-abrasives.com/
  [4]: https://sizes.com/tools/sandpaper.htm