Do you love coffee and strive for the perfect cup? Then you've certainly heard of grind size. But how "well" a grinder really grinds depends on much more than just whether the powder is fine or coarse.
This is where Particle Size Distribution (PSD) comes into play – a crucial factor for extraction and thus for the taste of your coffee. In recent years, we have intensively tested over 70 coffee grinders, delving deep into the world of particle analysis. In this article, we explain what particle distribution is all about, how we measure it, and which metrics are truly important.
Also worth reading: How Slow Feeding influences your grinder's particle distribution!
What is Particle Size Distribution (PSD)?
When your grinder breaks down coffee beans, it doesn't just produce a single particle size, but a whole spectrum – from very fine "fines" to coarser "boulders." Particle size distribution precisely describes this mix: How many particles of what size are in your grinds?
Typically, we see a so-called bimodal distribution in coffee grounds for espresso. This means there are usually two "peaks" in the distribution:
- A fines peak at very small particle sizes.
- A nominal peak, which accounts for the largest proportion of the grinds and whose position strongly depends on the set grind size.
This distribution is crucial because it significantly influences how water flows through the coffee during brewing and how quickly flavors are extracted. An uneven distribution can lead to uneven extraction – some particles may then be over-extracted (bitter), others under-extracted (sour).
How do we measure particle distribution? Our protocol
To obtain objective and comparable data, we work closely with the team at the Coffee Excellence Center of the Zurich University of Applied Sciences (ZHAW). Our measurements follow a standardized protocol:
- Measuring device: We use a high-precision measuring device based on dynamic image analysis (a Retsch Camsizer X2, affectionately known as “Kevin” internally). Coffee particles are passed through a chute and captured by cameras. Unlike laser diffraction, this not only gives us size but theoretically also shape information, even if we focus on size here.
- Sampling: A representative sample is essential. Since finer and coarser particles can segregate in the container, we use a sample divider. This mechanically and fairly divides the ground sample, so we get a small but representative amount for analysis.
- Standardization: All grinders are tested with the same coffee (a naturally processed Brazilian from the APAS cooperative) and according to defined recipes (T4 = Espresso, T5 = Ristretto, T6 = Lungo, T7 = back to Espresso) to ensure comparability.
The most important measured values in detail
We derive various key figures from the measurement data. The three most important for evaluating a grinder are:
x50 (Median):
x₅₀ (median particle size): This is the mean particle size in a statistical sense. More precisely: 50% of the particles are smaller than x₅₀ and 50% are larger. x₅₀ is therefore also called the median or D50. For espresso grinds, it typically lies in the several hundred micrometer range (e.g., 200–300 µm, depending on the grinder and setting). x₅₀ can be considered an approximate measure of the "average grind size" – it shifts to the left (smaller values) when we grind finer, and to the right (larger values) with a coarser grind size. For extraction, a small x₅₀ means: a lot of surface area, tending towards higher extraction (up to over-extraction) and slower flow. A larger x₅₀ results in less surface area, lower extraction (risk of under-extraction) and faster flow. Important: x₅₀ alone does not describe the entire distribution, but it is a good starting point for comparing grinds.
🔎 x50 in a nutshell: The x50 value (in micrometers, µm) is the point at which 50% of the volume of the grinds consists of particles smaller than this value, and 50% of particles that are larger. It represents the "typical" particle size in the grinds.
Fines content (Qf <100 µm / Fines Peak):
By this, we mean the fines content in the grinds – specifically, the percentage of particles that are smaller than 100 µm. Why 100 µm? Because in many grinder evaluations, there is a minimum point around this size between the large main peak and the "fine dust hill." Everything to the left of it we refer to as fines. This value (often given as Q_<100µm) thus indicates how dusty a grind is. Example: 30% <100µm means almost a third of the coffee particles are finer than a human hair's thickness – that's quite a lot of "coffee dust."
A high fines content can increase the body of an espresso (intensity; because more fine particles get into the cup), but also carries the risk of over-extraction and bitterness (the fines release flavor compounds very quickly and tend to over-extract). In addition, they clog the gaps in the puck and increase resistance: grinders that produce many fines often have to be set coarser for espresso to get a shot that flows even moderately well. Conversely, grinders with hardly any fines (so-called unimodal distributions with only one peak) can be set very finely without the shot stalling – because there are fewer clogging particles. The fines peak thus plays a decisive role in how a grind handles and tastes: from velvety and dense (lots of fine dust) to clear and light (little fine dust).
🔎 Fines content in a nutshell: This value indicates the percentage by volume of particles that are smaller than 100 micrometers. These very fine particles are often referred to as "fines" and form the fines peak in the distribution curve. This peak is often in the range of 30-70 µm.
"Main Peak Width" (60% Coarse Peak Width):
When we talk about the width of the main peak, we mean the size range in which 60% of all coarse particles (i.e., larger than 100 µm) are contained. The fines, i.e., the very fine particles below this threshold, are deliberately excluded. This provides a clear view of what a grinder produces in its "main business": the medium to coarse particles that significantly influence the espresso.
Let's imagine this main area as a mountain landscape without fog: Is the main peak a narrow, clear ridge, where almost all particles are of similar size? Or a sprawling hill surrounded by small hill ranges? This is precisely what the 60% Coarse Peak Width shows – it describes how narrow or wide the grind is distributed in the main area.
A small value means: The particle sizes are close together, the grind is uniform. A large value means: The distribution is broad – there are both smaller and significantly larger particles around the average. The consequence: Part of the coffee extracts faster, another slower – this can lead to taste imbalances.
Why does this matter? Because a narrow distribution suggests more even extraction – with balanced, clean flavors. If the distribution, however, is broad, medium-sized particles and coarse chunks ("boulders") often lie side by side in the puck. The latter release hardly any flavor – they under-extract, while the rest is already optimally or even over-extracted. The result: Acidity and bitterness stand unconnected next to each other, sweetness is lost.
In our tests with over 70 grinders, it has been shown: As soon as the main cloud becomes wider than about 300 µm, espressos often taste "scattered" – little structure, little harmony. A smaller peak width, on the other hand, is typical for modern grinders with precise cutting and stable alignment. They consistently produce good grounds – the basis for clear, extraction-stable espresso.
🔎 In short: The 60% Coarse Peak Width measures not the width at 60% of the curve height, but the size range in which 60% of the coarse particle mass is contained. It is an objective measure of the uniformity of the main component in the grinds – and thus a key indicator for the quality of the grinder.
Understanding Particle Size Distribution Curves
When we look at the results of particle measurement, we usually see two curves: an incremental and a cumulative distribution. Both show the same ground coffee sample – but from two different perspectives.
Important: The X-axis in both diagrams – i.e., the particle size – is logarithmically scaled. This means: The distance between 10 µm and 100 µm looks just as large as the distance between 100 µm and 1000 µm – although the second range covers ten times more size difference.
Why is this important? Because it can easily deceive our eyes:
- In the incremental curve, the area below 100 µm often appears wider than it actually is – although there is very little space there in reality.
- In the cumulative curve, a steep rise in the 200–300 µm range can suddenly appear much more dramatic, because the distance looks small visually, but is large in content.
In short: The X-axis is not linear, but follows a logarithmic division – this is necessary to clearly display both fine and coarse particles in one graph. But: It changes our perception of "area" and "weighting." Whoever reads the curve should keep this in mind.
Example of an incremental distribution.
Incremental Distribution (“Incremental Share”)
The incremental distribution (Incremental Share) looks like a small mountain range. It shows how many particles occur in a specific size range. The Y-axis indicates what proportion (in percent) of the total grinds lies within a narrow size interval – for example, between 240 and 250 µm. The higher a point on the curve, the more particles are precisely in this range. You immediately see: Where is the highest "mountain"? How many fines are there? And how widely is the whole thing spread out? This makes the incremental distribution the visual map of the grind size – it tells us where the majority of the particles are, how pronounced the fines peak is, and how homogeneous or scattered the grinds appear overall.
- The incremental curve shows you what percentage of the coffee powder falls into a specific size range (e.g., how much is between 30-100 µm).
- It visualizes the peaks very clearly – you recognize the fines peak and the main peak and see whether the distribution is bimodal. The X-axis (particle size) is often logarithmically displayed here to make the fine range more visible.
- If there is a value of 2% on the Y-axis at 250 µm (X-axis), this means: → Approximately 2% of the total measured coffee consists of particles that are approximately 250 µm in size (more precisely: within the measurement interval around 250 µm).
Example of a cumulative distribution
Cumulative Distribution (“Cumulative Share”)
The cumulative distribution (Cumulative Share) tells the same story – but in a different way. Here, the Y-axis shows how much percentage of the coffee has already been reached if we sum up all particles up to a certain size. The curve starts at zero on the left and then rises steadily – until it ends at 100% on the right. The changes in slope are particularly interesting: A steep section means that many particles are present in this size range – a lot is happening in the coffee there. A flat section means: Little is happening here, only a few particles move in this size range. You could say: The cumulative curve shows how quickly the bag of coffee fills up when we fill it with particles from fine to coarse.
- This curve shows the total proportion (in percent) of all particles that are smaller than or equal to a certain size.
- It always rises from 0% to 100%.
- From this curve, you can easily read the x50 value (where the curve crosses the 50% line) and also the fines content (the Y-value at 100 µm on the X-axis).
Why these values influence taste
Now it gets exciting: How do these distribution properties manifest in the cup? Both empirical experience and scientific findings come into play here. On the one hand, experienced tasters quickly notice if an espresso, for example, tastes over-extracted and bitter due to too many fines, or remains watery and sour due to overly coarse particles. On the other hand, analyses – for example, by astrophysicist and coffee researcher Jonathan Gagné – have clearly shown that when dialing in an espresso, we are essentially adjusting the fines content. In an evaluation of 24 espresso grinders from our test series, Gagné found that different grinders produce surprisingly similar amounts of fines at optimal settings – regardless of the mean particle diameter. In other words: Baristas mainly adjust the grind size until the total amount of fines is suitable to achieve the desired flow and pressure.
A "fines-friendly" grinder must therefore be set much coarser (so that not too many fines clog the puck), while a "low-fines" grinder can be set very finely (to build up enough resistance in the puck at all). This interplay explains why, for example, unimodal grinders (with few fines) often require very fine shots, which then deliver exceptionally clear flavor notes – one often speaks of "low-fines shots" that emphasize brighter acids and floral notes. Conversely, grinders with deliberately more fines often yield stronger, more full-bodied espressos with a thicker texture – classically Italian with more bitter chocolate character, but sometimes also rougher in acidity. The width of the main peak also reflects in the taste. In our tests, we repeatedly found that very broad distributions (high main peak width) lead to complex flavor combinations: A certain unrest in the cup, one might say, which we also described as "scattered." Part of the extraction goes too far (bitterness, sometimes a metallic note), another remains behind (sharp acidity), and it is difficult to achieve a balanced overall taste.
If, on the other hand, the particle sizes are more homogeneous (narrower peak), the taste usually harmonizes better – sweetness, acidity, and bitterness are in balance, nothing stands out unpleasantly. This does not mean that every grinder with many fines or a broad distribution automatically makes bad coffee. Taste judgments depend on many factors (bean variety, roast, recipe, etc.), and much can be compensated for with technique. Nevertheless: The trends are clear. Grinders that grind very uniformly are appreciated by many coffee nerds for their clear, balanced shots. Grinders that produce more fines often yield strong, dense shots, but these are more difficult to perfectly balance. Here, personal taste also plays a role: Some like the chocolatey power of a slightly "dirty" espresso, others love the transparent nuance of a "clean" espresso – both can be excellent in their own way. However, particle distribution gives us the scientific tools to make such characteristics of a grinder tangible and to work with them specifically.
Of course, these three values are also related: Grinders with many fines (high fines content) often produce a somewhat broader distribution and a larger x₅₀ because they have to be set coarser. Conversely, grinders with few fines often have a smaller x₅₀ (must be set finer) and a narrower main peak. Nevertheless, it is worthwhile to consider each characteristic individually to understand the overall picture.
Conclusion: What we can learn from particle analysis for the perfect coffee
Dealing with particle distributions impressively shows that in the seemingly simple act of coffee grinding, there is a hidden world of science. For us coffee lovers, this means: We can approach the matter more consciously. If we understand that a grinder not only grinds "fine" or "coarse," but produces an individual fingerprint of fines, average particles, and perhaps a few boulders, then we can better adjust our coffee recipes accordingly – or choose the grinder that suits our preferences.
Different burr geometries of the same size with different designs such as cut, pre-breaker, and narrowness of the cutting edges also significantly change the particle distribution.
The insights from particle analysis inspire us to perhaps take a closer look (or taste) at our next espresso: Do I taste any hints that my grinder produces many fines? Is the espresso velvety, heavy, perhaps with a certain dryness in the aftertaste and less clarity in the flavors? Then this could be an indication of a high fines content.
Or is it crystal clear in its complexity and the individual notes of the coffee stand out – but the body is somewhat less pronounced and perhaps juicy and not heavy and dense? Then this is possibly a sign of a very homogeneous, low-fines grind.
Ultimately, the particle distribution shows us how closely craft and science are linked in coffee. The best results come when we use both: the curiosity and experimental spirit of the home barista and the knowledge from research. Our journey through the world of coffee particles has shown that behind every aromatic cup lies a lot of physics and statistics – but don't worry: you don't have to be an astrophysicist to benefit from it (even if people like Jonathan Gagné certainly help!).
Even a basic understanding of what happens in coffee grounds can help us make more informed decisions. Be it investing in a particular grinder or fine-tuning our recipe – scientific measurements like particle analysis give us an objective basis to better understand the myth and magic of espresso.
In the end, it's about the taste in the cup. Particle distribution is not an end in itself, but a key that helps us unravel and control taste. So the next time we talk about "coarse" or "fine," let's remember: there's much more to it – an entire world of particles waiting to be discovered. Have fun exploring further and on your way to the perfect coffee!
