What are coffee fines? Why tiny particles change your brew

How small is a fine?

A fine is usually defined as a particle smaller than 100 µm, roughly a tenth of a millimeter. The Smrke, Eiermann and Yeretzian study measured fines as the volume share of particles under that size.

The cutoff is a convention rather than a law of nature. Kaffeemacher, the Swiss grinder testers who supplied the samples for Coffee ad Astra's 24-grinder dataset, explains the choice: ground coffee tends to show a dip around 100 µm between the large main peak and a smaller "hill" of fine dust. Drawing the line at that dip separates the two populations cleanly.

For the full picture of how fines sit within the rest of the grind, see grind size distribution.

Are fines bad for coffee?

Fines are not simply bad; they are a lever that changes how water moves through the coffee. In the 2024 espresso study, the authors concluded that within the range they tested, fines were "from a practical standpoint only modifying coffee bed permeability." In other words, they mostly changed flow rather than changing extraction chemistry directly.

That flow effect still shapes the cup. A slower flow means a longer contact time, which pushes extraction up. Coffee ad Astra also notes that smaller particles extract faster as a group, because the same mass of coffee presents far more surface area when it is finely divided.

Coffee ad Astra argues that a wide spread of sizes mixes several flavor profiles in one cup. Its view is that very uneven grinds force you to stop at a lower average extraction to avoid astringency, while more uniform grinds let you push further.

Why do fines slow down a pour-over?

Fines can migrate down through the coffee bed and collect at the filter, where they block pores. Coffee ad Astra describes how that accumulation can "cause clogging" and slow the water "in a poorly controlled way," which also makes the flow less even across the bed.

The same article explains that particle movement in a bed works through a convection-like circulation driven by friction against the walls. Cone-shaped brewers like the V60 reverse the direction of that circulation. It still does not prevent clogging, because fines small enough to enter the paper's pores get stuck there.

If your drawdown times wander from brew to brew at the same setting, fines are one suspect. The pour-over dial-in guide covers how to adjust around it.

Do flat burrs make fewer fines than conical burrs?

On average the measured data leans that way, but the overlap is large and the taste question is contested. Coffee ad Astra's analysis of about 300 distributions from 24 espresso grinders found that conical burrs were "on average less unimodal and less uniform" than flat burrs.

The same analysis warns that "one can easily find a specific flat burr that is less unimodal and less uniform than a specific conical burr." So burr shape alone does not tell you how many fines a given grinder will make. The individual burr design, its alignment and the grinder's build all play a part.

Whether fewer fines tastes better is also not settled, and the tasting evidence is thin.

Does a grinder with fewer fines give you fewer fines in the cup?

Not necessarily, which surprises many buyers. Kaffeemacher, summarizing Coffee ad Astra's analysis, reports that different grinders, each dialed in to their best setting, produced "surprisingly similar amounts of fines," regardless of average particle size.

Coffee ad Astra goes further: grinders that make fewer fines at a given setting tended to need a slightly higher share of fines to be dialed in. The author says he finds this over-shooting hard to explain, and suspects indirect effects such as differences in how the puck compresses.

The practical point is that you dial in by taste and flow, and the grinder's fines tendency changes where you end up on the dial. The Niche Zero, one of the 24 grinders in that dataset, is a good example of a model people judge by the cup rather than the chart.

What should you look for when choosing a grinder?

Look at independent particle measurements where they exist, and treat them as one input rather than a verdict. Coffee ad Astra found that the top-rated grinders in blind tasting sat in the middle of its uniformity chart rather than at one extreme, and that other factors seem to be involved. One poorly rated grinder measured close to the top performers, which the author suggested might come down to clumping or heating of the grounds.

The Smrke study adds that the balance between the fines peak and the main peak depends on grind size and is "indicative of the grinding technology." That is a reason to compare like with like, at the same brew method, rather than across methods.

When you are down to two models, compare their settings charts side by side to see where each one puts your brew method on its dial.