How is sandpaper grit size (the FEPA number system) defined?

Question

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. 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. 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 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, explain how to understand the standard but not how the number relates to a physical value. The Wikipedia article 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 is of no help either since it seems to require payment for the information.

Thank you for you help.

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Update

I have been looking into sandpaper types.

• FEPA P220 equals CAMI 220. Labeled grain size: $68\;\mathrm{\mu m}$
• FEPA P500 equals CAMI 360. Labeled grain size: $30\;\mathrm{\mu m}$
• FEPA P1000 equals CAMI 500. Labeled grain size: $18\;\mathrm{\mu m}$
• FEPA P4000 equals CAMI 1200. Labeled grain size: $5\;\mathrm{\mu m}$

If the number is the number of grains (e.g. 220 grains when the number is P220) that stacked reach some height (for example 1 inch which is 2.54 cm) lengths would be:

• The P220 gives ~1.5 cm and the CAMI 220 gives 1.5 cm.
• The P500 gives 1.5 cm and the CAMI 360 gives 1.08 cm.
• The P1000 gives 1.8 cm and the CAMI 500 gives 0.9 cm.
• The P4000 gives 2 cm and the CAMI 1200 gives 0.6 cm.

There is no system and none come close to for example one inch (not even the American system). Otherwise there must be gaps and spaces counted in as well that I am not aware of. From these quick calculations, the number does not seem to be a "number of grains per length" value or alike.

Answer 1

In general grit numbers refer to the size of mesh that the particles will pass through. There are various ways of indicating this but it is typically some variation of the number of strands per unit length of a woven mesh.

This is generally defined as strands per inch, even if it is part of an otherwise metric system, in which case it will be technical strands per 25.4mm.

So P36 grit would pass through a square grid with 36 lines per inch ie a hole 0.706 mm square.

This is based on the assumption that the resulting grit would also not pass through the next mesh in the sequence.

This isn't necessarily how abrasives are actually made or sorted but is the basis of the standard.

Answer 2

If you check out the wiki entry on mesh size, the deviations become more aparent.

Both follow the mesh size scheme. However, there are two norms for macro and micro grids, denoted by Z and P in the European norm for example. Furthermore, since mesh size first and foremost designates the particle size that falls through a grid with a certain number of wires per length. This value disregards the wire diameter and hence the actual space is defined differently in different standard mesh norms. On top of that, due to usually grains will have some kind of size distribution. So the actual mesh number is then defined as a certain percentage of grains passing through a certain mesh size, but not the next.

As a result, actual grain size ranges vary across size ranges, as well as norms, instead of progressing linearly.