Smaller grind particles will always be at a high extraction as they have so much surface area and so little volume. When water touches them, almost everything they have to offer extracts immediately.
Larger grind particles will almost always be at a lower average extraction, unless they’ve been brewed for a long time. Their complex internal structure makes it difficult for water to travel through. Water needs to get into the grinds, extract the flavour, and take the flavour back out again. The internal volume of large coffee grinds will mostly go untouched in all but the longest brews.
Here is a typical graph of coffee grinds from a high-quality filter coffee grinder and a less perfect domestic grinder.
Along the x-axis (horizontal) is particle size: larger to the right, smaller to the left. For scale, 1000 microns (um) equals 1 millimetre (mm).
Along the y-axis (vertical) is percentage volume. When you assume that every grind is a sphere, then it’s quite easy to figure out how much volume they have. To find the percentage volume you add up the collective volume of every particle at a particular size, then calculate the percentage of the total volume of all the grinds that they represent.
Volume = 4/3 x pi x radius cubed.
For example, we compare the two lines at 400um, you’ll notice the red line indicates 1% volume and the blue line 1.4% volume. This means there are 40% more 400um particles produced by the home grinder.
It’s easiest to think of this graph simply as how much of each size there is. If you follow the curve from left to right, you’ll find very few single micron-sized particles, and a fair few tiny particles we call fines between zero and 50um.
Note: We consider a fine to be any particle that graphs to the left of the main particle size group. In practice, this is usually smaller than 100um.