One of the biggest problems when describing a brewing method is getting across what size of ground coffee to use. The trouble is that each grind setting on a coffee grinder will produce a range of sizes rather than one exact size.

How coffee grinders work

The reason we get a range of sizes from a grinder, is that grinders use a crushing action to break the bean into smaller pieces. On a conical burr grinder, there are two discs: a female cylindrical disc and a male dome shaped disc. Each disc has ridges (know as teeth) running across its surface, and it’s these ridges which crush the bean. Obviously crushing isn’t an exact science. Irregular sized pieces break off from the bean each time it’s crushed between the grinder’s teeth.

If you look at the cross section below of a burr grinder: the bean enters at the top and as it progresses down, the gap narrows between the two discs, breaking the bean into smaller and smaller pieces. The ground coffee cannot escape from between the grinding discs until it’s small enough to fit through the gap at the bottom (indicated by the two red arrows).

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