You love coffee and strive for the perfect cup? Then you've surely heard about grind size. But how "well" a grinder really grinds depends on far more than just whether the powder is fine or coarse.
This is where particle size distribution (Particle Size Distribution = PSD) comes into play – a decisive factor for extraction and thus for the taste of your coffee. Over the past few years, we have intensively tested over 70 coffee grinders and delved deep into the world of particle analysis. In this article, we explain what particle distribution is all about, how we measure it, and which key metrics really matter.
Also Worth Reading: How Slow Feeding affects your grinder's particle distribution!
What is a particle distribution (PSD)?
When your grinder breaks down coffee beans, it doesn't create just a single particle size, but rather a whole spectrum – from very fine "fines" to coarser "boulders". Particle size distribution describes exactly this mixture: How many particles of which size are in your ground coffee? How this spectrum turns out depends heavily on the grinding mechanism, so whether you're grinding with a conical burr or flat burr grinder.
Typically, we see what's called a bimodal distribution in espresso grounds. This means there are usually two "peaks" in the distribution:
- A fine peak (Fines Peak) at very small particle sizes.
- A main peak (Nominal Peak), which makes up the largest portion of the grounds and whose position depends heavily on the set grind size.
This distribution is crucial because it significantly influences how water flows through the coffee during brewing and which flavors are extracted and how quickly. An uneven distribution can lead to uneven extraction – some particles are then 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 Zurich University of Teacher Education (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, internally affectionately called "Kevin"). Coffee particles are guided through a shaft and captured by cameras. Unlike laser diffraction, this allows us to theoretically obtain not just size information, but also shape information, although we focus on size here.
- Sampling: A representative sample is essential. Since finer and coarser particles can separate 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
From the measurement data, we derive various key metrics. The three most important for evaluating a grinder are:
x50 (Median):
x₅₀ (Median particle size): This is the average particle size in the statistical sense. More precisely: 50% of particles are smaller than x₅₀ and 50% are larger. Therefore x₅₀ is also called the median or D50. For espresso grinds, it typically falls in the hundreds of micrometers range (e.g., 200–300 µm, depending on grinder and setting). x₅₀ can be considered an approximate measure of the "average grind size" – it shifts left (smaller values) when we grind finer, and right (larger values) when grinding coarser. For extraction, a small x₅₀ means: lots of surface area, potentially higher extraction (up to over-extraction) and slower flow. A larger x₅₀ yields less surface area, lower extraction (risk of under-extraction) and faster flow. Important: x₅₀ alone doesn't describe the entire distribution, but it's 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 ground coffee volume consists of particles smaller than this value, and 50% consists of particles larger. It represents the "typical" particle size in the grounds.
Fines content (Qf <100 µm / Fine peak):
By this we mean the fines content in the grounds – specifically the percentage of particles smaller than 100 µm. Why 100 µm? Because in many grinder evaluations, there's a minimum point around this size between the large main peak and the "dust hill". Everything to the left of this we call fines. This value (often given as Q_<100µm) tells us how dusty a grind is. Example: 30% <100 µm means almost a third of coffee particles are finer than a human hair is thick – that's quite a lot of "coffee dust".
A high fines content can increase the body of an espresso (intensity; because more fine particles reach the cup), but also carries the risk of over-extraction and bitterness (the fines release flavors very quickly and tend toward over-extraction). Furthermore, they clog spaces in the puck and increase resistance: grinders that produce many fines often need to be set coarser for espresso to achieve a reasonably flowing shot. Conversely, grinders that produce few fines (so-called unimodal distributions with only one peak) can be set much finer without the shot drying up – because there are fewer clogging particles. The fine peak therefore decisively determines how grounds handle and taste: from silky and dense (lots of dust) to clear and light (little dust).
🔎 Fines content in a nutshell: This value indicates the percentage volume share of particles that are smaller than 100 micrometers. These very fine particles are often called "fines" and form the fine peak in the distribution curve. This peak often lies in the range of 30-70 µm.
"Main peak width" (60% Coarse Peak Width):
When we speak of 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, meaning the very fine particles below this threshold, are intentionally excluded. This creates a clear view of what a grinder produces in its "main business": the medium to coarse particles that significantly influence espresso.
Imagine this main area as a mountain landscape without fog: Is the main peak a narrow, clear ridge, where almost all particles have similar size? Or a sprawling hill surrounded by small hillsides? That's exactly what the 60% Coarse Peak Width shows – it describes how narrow or wide the grounds distribute in the main area.
A small value means: The particle sizes lie close together, the grounds are uniform. A large value means: The distribution is wide-ranging – there are both smaller and noticeably larger particles around the average. The consequence: Part of the coffee extracts faster, another part slower – this can lead to taste inconsistency.
Why does this matter? Because a narrow distribution suggests more uniform extraction – with balanced, clean flavors. If the distribution is broad, medium-sized particles and coarse "boulders" often sit side by side in the puck. The latter give off little flavor – they under-extract, while the rest are already optimal or even over-extracted. The result: Acidity and bitterness stand disconnected side by side, the sweetness is lost.
In our tests of over 70 grinders, it became clear: Once the main cloud gets wider than about 300 µm, espressos often taste "scattered" – little structure, little harmony. A narrower peak width, on the other hand, is typical of modern grinders with precise cutting and stable alignment. They consistently produce quality grounds – the foundation for clean, extraction-stable espresso.
🔎 In short: The 60% Coarse Peak Width doesn't measure width at 60% of curve height, but rather the size range in which 60% of the coarse particle mass is contained. It's an objective measure of the uniformity of the main portion in the grounds – and thus a key indicator of grinder quality.
Understanding particle 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 sample – but from two different perspectives.
Important: The X-axis in both diagrams – particle size – is logarithmically scaled. This means: The distance between 10 µm and 100 µm looks as large as the distance between 100 µm and 1000 µm – even though the second range encompasses ten times the size difference.
Why is this important? Because it easily tricks our eye:
- In the incremental curve, the area under 100 µm often looks wider than it actually is – even though there's actually very little space there.
- In the cumulative curve, a steep rise in the 200–300 µm range can suddenly appear much more dramatic because the distance looks optically small but is substantially large.
In short: The X-axis is not linear, but follows a logarithmic scale – this is necessary to represent both fine and coarse particles clearly in one graph. But: It changes our perception of "area" and "weighting". Anyone reading 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 certain size range. The Y-axis indicates what share (in percent) of the total grounds lies within a narrow size interval – for example between 240 and 250 µm. The higher a point on the curve, the more particles are in exactly this range. You immediately see: Where is the highest "mountain"? How many fines are there? And how broad is the whole thing spread? This makes the incremental distribution the visual map of your grind – it tells us where the majority of particles lie, how pronounced the fine peak is, and how homogeneous or scattered the grounds appear overall.
- The incremental curve shows you what percentage of coffee powder falls into a specific size range (e.g., how much lies between 30-100 µm).
- It visualizes the peaks very clearly – you recognize the fine peak and main peak and see whether the distribution is bimodal. The X-axis (particle size) is often shown logarithmically here to make the fine range more visible.
- If at 250 µm (X-axis) there's a value of 2% on the Y-axis, that means: → Approximately 2% of all measured coffee consists of particles that are approximately 250 µm large (more precisely: in 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 what percentage of coffee is already included when we sum up all particles up to a certain size. The curve starts on the left at zero and then rises steadily – until it reaches 100% on the right. The slope changes are particularly interesting: A steep section means that many particles are present in this size range – something real is happening in the coffee there. A flat section means: Not much happening here, only a few particles move in this size range. You could say: The cumulative curve shows how quickly the coffee bag fills up when we fill it with particles from fine to coarse.
- This curve shows the total share (in percent) of all particles that are less 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 interesting: How do these distribution properties show up in the cup? Both empirical experience and scientific findings come into play. On one hand, trained tasters quickly notice when an espresso, for example, tastes over-extracted-bitter due to too many fines or remains watery and sour due to too-coarse particles. On the other hand, analyses – such as by astrophysicist and coffee researcher Jonathan Gagné – have clearly shown that when dialing in an espresso, we're basically adjusting the fines content. In an evaluation of 24 espresso grinders from our measurement series, Gagné found that different grinders at optimal settings produce remarkably similar amounts of fines – regardless of the average particle diameter. In other words: Baristas turn the grind size mainly until the total amount of fines is right to achieve the desired flow rate and pressure.
A "fines-friendly" grinder therefore needs to be set much coarser (so that too many fines don't clog the puck), while a "fines-poor" grinder can be set very fine (to build up enough resistance in the puck at all). This interaction explains why, for example, unimodal grinders (with few fines) often need very fine shots, which then deliver exceptionally clear flavor notes – people often speak here of "low-fines shots" that emphasize lighter acidity and floral notes. Conversely, grinders with deliberately somewhat more fines often deliver more intense, fuller-bodied espressos with thicker texture – classically Italian with more bitter-chocolate character, but sometimes also rougher in acidity. The width of the main peak is also reflected in the taste. In our tests, we repeatedly found that very broad distributions (high Main Peak Width) lead to complicated flavor combinations: a kind of unrest in the cup, you might say, which we've also described as "scattered". Some of the extraction goes too far (bitterness, sometimes a metallic note), some falls short (sharp acidity), and it's difficult to achieve a balanced overall taste.
When particle sizes are more homogeneous (narrower peak), however, the flavor usually comes together more harmoniously – sweetness, acidity, and bitterness are in harmony, nothing stands out unpleasantly. This doesn't 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 you can compensate for a lot with technique. Nevertheless: The trends are clear. Grinders that grind very uniformly are valued by many coffee enthusiasts for their clean, balanced shots. Grinders that produce more fines often yield intense, dense shots, but are harder to get perfect. Here, personal taste also plays a role: Some like the chocolatey punch 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 grinder characteristics tangible and to work with them intentionally.
Of course, these three values are also related to each other: Grinders that produce many fines (high fines content) often create a somewhat broader distribution and a larger x₅₀ because you need to set them coarser. Conversely, grinders with few fines often have a smaller x₅₀ (need to be set finer) and a narrower main peak. Still, it's worth looking at each metric individually to get the complete picture.
Conclusion: What we can learn from particle analysis for perfect coffee
Examining particle distributions vividly shows that in what seems like the simple act of grinding coffee, there's a hidden world of science. For us coffee lovers, this means: We can approach it more consciously. When we understand that a grinder doesn't just grind "fine" or "coarse", but produces an individual fingerprint of fines, average particles, and maybe a few boulders, we can better adjust our coffee recipes to it – or choose the grinder that suits our preferences.
Different grinding discs of the same size with different geometries like cutting, pre-breaking, and tightness of cutting edges also greatly change the particle distribution.
The insights from particle analysis inspire us to look (or taste) more carefully at the next espresso: Do I detect signs that my grinder produces many fines? Is the espresso silky, heavy, perhaps with a certain dryness in the aftertaste and less clarity in the flavors? Then that 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 – the body is however somewhat less pronounced and perhaps juicy and not heavy and dense? Then that's possibly a sign of very homogeneous, fines-poor grinding.
Ultimately, particle distribution shows us how closely craftsmanship and science are linked in coffee. The best results come when we use both: the curiosity and experimental enthusiasm of the home barista and the knowledge from research. Our journey through the world of coffee particles has shown that behind every aromatic cup is a lot of physics and statistics – but don't worry: You don't need to be an astrophysicist to benefit from it (though people like Jonathan Gagné certainly help!).
Even a basic understanding of what happens in coffee grounds can help us make more conscious decisions. Whether it's investing in a particular grinder or fine-tuning our recipe – scientific measurements like particle analysis give us an objective foundation to better understand the myth and magic of espresso.
Ultimately, it's about the taste in the cup. Particle distribution isn't an end in itself, but a key that helps us decipher and control taste. So the next time we talk about "coarse" or "fine", let's remember: there's much more to it – an entire particle world waiting to be discovered. Happy exploring and on your way to perfect coffee!
