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Grain sizes: micron, JIS or FEPA?

Micron, JIS, FEPA-F, FEPA-P... Any idea what we are talking about? They all represent the grain sizes of sharpening stones. Important terms as the grain size determines how coarse or fine your sharpening stone is. A larger grain means you have a coarser stone which removes more material. A smaller grain, obviously, means the complete opposite.

As such these grain sizes can be indicated differently. The result is that it can be difficult to determine how fine or coarse a specific stone really is. Some suppliers prefer to give the absolute grain size in microns, others adhere to standard European sizes and yet another prefers the Japanese version. We understand that, as a result, it could become really difficult to select the right sharpening stone.

Micron, JIS or FEPA?

It is important to realize that the size of the grain sizes can be expressed in absolute sizes and standardized sizes. The absolute size of a grain is a so-called 'ratio-variable': a measurable number, where a 0-value means the absence of the variable. To determine the size of the grain sizes microns are used. A micron is a thousandth of a millimetre and can therefore not be detected with the naked eye. Essential for a ratio- (or interval) variable is that a doubling of the value (for example from one to two micron) also means that this grain doubles in size.

Standardized sizes, on the other hand, are nominal variables. This means that the number that is used to express the size doesn't have a unit of measurement, but is only a name. Hence the name 'nominal variable', from the Latin nomen (=name). Such variables do not have an absolute zero point and are not measurable. A standardized 16.000 grain is, as a result, not necessarily twice as large as a 8000 grain.

European and Japanese standard sizes

There are multiple standard versions for grain sizes. Officially recognized are the European FEPA (Federation of European Producers of Abrasives) and the in Japan adhered to JIS (Japanese Industrial standards). Within the FEPA sizes you can also make a distinction between FEPA-P and FEPA-F. The P-version represents the grain size of sand paper and will therefore not be discussed here. FEPA-F is used to determine the grain size of sharpening stones and is therefore relevant.

Because many sharpening stones come from Japan, the JIS is still a frequently used way of determining the 'grit' of whetstones. However, even Japanese manufacturers start using the European version. As a result it can sometimes be difficult to compare the sharpening stones from different manufacturers, especially when you don't realize that (in particular American) manufacturers also express their grain sizes in absolute microns. For that reason we listed the three most used sizes in the table below. Hopefully this will make the choice a little easier!