You love coffee and strive for the perfect cup? Then you've surely already 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 in – a crucial 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 post, we explain what particle distribution is all about, how we measure it, and which 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 a whole spectrum – from very fine "fines" to coarser "boulders". The 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, i.e., whether you're grinding with a conical or flat burr grinder.
Typically, we see a so-called bimodal distribution in espresso grounds. This means there are usually two "peaks" in the distribution:
- A fine 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 the Zurich University of Teacher Education (ZHAW). Our measurements follow a standardized protocol:
- Measurement device: We use a highly precise measurement device based on dynamic image analysis (a Retsch Camsizer X2, affectionately called "Kevin" internally). Coffee particles are fed through a shaft and captured by cameras. Unlike laser diffraction, we thus obtain not only size but theoretically 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 divides the ground sample mechanically and fairly so that we obtain a small but representative amount for analysis.
- Standardization: All grinders are tested with the same coffee (a natural-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 metrics. The three most important for evaluating a grinder are:
x50 (Median):
x₅₀ (median particle size): This is the middle particle size in the statistical sense. More precisely: 50% of the particles are smaller than x₅₀ and 50% are larger. Therefore, x₅₀ is also called the median or D50. For espresso grinds, it typically lies in the hundreds-of-micrometers range (e.g., 200–300 µm, depending on grinder and setting). x₅₀ can be regarded as a rough measure of the "average grind size" – it shifts to the left (smaller values) when we grind finer, and to the right (larger values) with coarser grinding. For extraction, a small x₅₀ means: lots of surface area, potentially 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 grounds consists of particles smaller than this value, and 50% of particles larger than it. It represents the "typical" particle size in the grounds.
Fine content (Qf <100 µm / Fine peak):
By this we mean the fine content in the grounds – specifically the percentage of particles 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 this we call fines. This value (often listed as Q_<100µm) thus tells you how dusty a grind is. Example: 30% <100 µm means almost a third of the coffee particles are finer than a human hair is thick – that's quite a lot of "coffee dust".
A high fine content can increase the body of an espresso (intensity; because more fine particles make it into the cup), but also carries the risk of over-extraction and bitterness (the fines release flavor compounds very quickly and tend toward over-extraction). Additionally, they clog gaps in the puck and increase resistance: grinders that produce many fines often need to be set much coarser for espresso to get a reasonably flowing shot. Conversely, grinders that produce hardly any fines (so-called unimodal distributions with only one peak) can be set much finer without the shot dying – because there are fewer clogging particles. The fine peak thus plays a decisive role in how a grind handles and tastes: from silky and dense (lots of fine dust) to clear and light (little fine dust).
🔎 Fine 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, i.e., the very fine particles below this threshold, are deliberately excluded. This creates a clear view of what a grinder produces in its "main business": the medium to coarse particles that primarily influence espresso.
Imagine this main range as a landscape without fog: Is the main peak a narrow, clear ridge where almost all particles are similar in size? Or a sprawling hill surrounded by small hills? That's exactly what the 60% Coarse Peak Width shows – it describes how narrow or broad the grind is distributed in the main range.
A small value means: The particle sizes are close together, the grind is uniform. A large value means: The distribution is widely spread – there are both smaller and significantly larger particles around the average. The consequence: Some of the coffee extracts faster, some slower – this can lead to flavor inconsistency.
Why does this matter? Because a narrow distribution suggests more uniform extraction – with balanced, clean flavors. If the distribution is broad, you often find medium-sized particles and coarse boulders lying side by side in the puck. The latter give off little flavor – they under-extract, while the rest already extracts optimally or even too far. The result: Sourness and bitterness stand disconnected side by side, sweetness is lost.
In our tests with over 70 grinders, it has become clear: Once the main cloud becomes broader 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 burrs and stable alignment. They consistently produce good grinds – the foundation for clear, extraction-stable espresso.
🔎 In short: The 60% Coarse Peak Width does not measure the width at 60% of curve height, but rather the size range in which 60% of the coarse particle mass is contained. It is 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 coffee sample – but from two different perspectives.
Important: The X-axis in both diagrams – i.e., particle size – is logarithmically scaled. This means: The distance between 10 µm and 100 µm looks the same as the distance between 100 µm and 1000 µm – even though the second range encompasses 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 looks broader than it actually is – even though there is 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 small visually, but is large in content.
In short: The X-axis is not linear, but follows a logarithmic scale – this is necessary to display both fine and coarse particles clearly in one graph. But: It changes our sense 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 specific size range. The Y-axis indicates what percentage of the total grounds falls 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 can immediately see: Where is the highest "mountain"? How many fines are there? And how broadly is everything spread? This makes the incremental distribution the visual map of the grind size – it reveals where the majority of particles lie, how pronounced the fine peak is, and how homogeneous or scattered the grind is overall.
- The incremental curve shows you what percentage of the coffee powder falls within 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 displayed logarithmically here to make the fine range more visible.
- If at 250 µm (X-axis) a value of 2% is on the Y-axis, that means: → Approximately 2% of the entire measured coffee consists of particles that are roughly 250 µm in size (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 the coffee has already been reached if we add 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. Particularly interesting are the changes in slope: A steep section means that many particles are present in this size range – something real is happening with the coffee there. A flat section means: There's not much 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 if we fill it with particles from fine to coarse.
- This curve shows the total percentage (in percent) of all particles that are smaller than or equal to a specific 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 fine 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 translate into the cup? Both empirical experience and scientific findings come into play here. On one hand, experienced tasters quickly notice when an espresso, for example, due to too many fines, tastes over-extracted-bitter or due to too-coarse particles remains watery and sour. On the other hand, analyses – such as by astrophysicist and coffee researcher Jonathan Gagné – have clearly shown that when we dial in an espresso, we're essentially adjusting the fine content. In an evaluation of 24 espresso grinders from our test series, Gagné found that different grinders at optimal settings produce surprisingly similar amounts of fines – regardless of the average particle diameter. In other words: baristas turn the grind setting primarily until the total amount of fines is right to achieve the desired flow and pressure.
A "fines-loving" grinder must therefore 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 interplay explains why, for example, unimodal grinders (with few fines) often require very fine shots, which then deliver exceptionally clear flavor notes – one likes to speak of "low-fines shots" here, which emphasize lighter acidity and floral notes. Conversely, grinders with deliberately more fines often produce fuller, more full-bodied espressos with thicker texture – classically Italian with more bitter-chocolate character, but sometimes rougher in the acidity too. The width of the main peak is also reflected in taste. In our tests, we repeatedly found that very broad distributions (high main peak width) result in complicated flavor combinations: a certain unrest in the cup, you could say, which we also described as "scattered". Some of the extraction goes too far (bitterness, sometimes a metallic note), some falls short (sharp sourness), and it's hard to achieve a balanced overall taste.
When particle sizes become more homogeneous (narrower peak), on the other hand, the taste 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 balance a lot with technique. Nevertheless: the trends are clear. Grinders that grind very uniformly are appreciated by many coffee enthusiasts for their clean, balanced shots. Grinders that produce more fines often yield strong, dense shots, but are more difficult to dial in perfectly. Personal taste also plays a role here: 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 characteristics of a grinder tangible and work with them deliberately.
Of course, these three values are also related to each other: Grinders that produce many fines (high fine content) often create a slightly broader distribution and a larger x₅₀ because you have to set them coarser. Conversely, grinders with few fines often have a smaller x₅₀ (need to be set finer) and a narrower main peak. Nevertheless, it's worth considering each metric individually to understand the full picture.
Conclusion: What we can learn from particle analysis for the perfect coffee
Engaging with particle distributions impressively shows that in the seemingly simple act of grinding coffee, there is a hidden world of science. For us coffee lovers, this means: We can approach the subject more consciously. When we understand that a grinder doesn't just grind "fine" or "coarse", but creates an individual fingerprint of fines, average particles, and perhaps a few boulders, then we can adjust our coffee recipes better for it – or choose the grinder that matches our preferences.
Different grinding discs of the same size with different geometries such as cut, pre-breaker, tightness of cutting edges also change the particle distribution very significantly.
The insights from particle analysis inspire us to perhaps look more closely at the next espresso (or taste it): Do I taste hints that my grinder produces many fines? Does the espresso taste 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 fine 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 that might be a sign of very homogeneous, fine-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 willingness to experiment of the home barista and knowledge from research. Our journey through the world of coffee particles has shown that behind every aromatic cup there is a lot of physics and statistics – but don't worry: you don't have to be an astrophysicist to benefit from it (although 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 basis to understand the myth and magic of espresso a little better.
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 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. Have fun exploring further and on your way to the perfect coffee!
