← Coffee guide

Coffee grind distribution: boulders, fines and uniformity

When choosing a coffee grinder, most people think only of one thing: how coarsely it grinds. But there is a second, equally important feature that is rarely talked about: the uniformity of the grind, that is the distribution of particle sizes. Even at the same coarseness setting, a grinder never produces perfectly even particles - alongside particles of the target size appear boulders (too large pieces) and fines (too small particles, dust). The more of these extremes, the worse the coffee brews: too large pieces under-extract (sour), too small ones over-extract (bitter). This is why a good grinder is one that grinds uniformly. Here is a guide to grind distribution: what boulders and fines are, why uniformity is key and how conical burrs differ from flat ones.

Grinding is not only about coarseness

Let us begin by debunking a simplification: grinding coffee is not only a matter of coarseness. Yes, you set the grinder to a coarser or finer grind - that is the basis. But it is only half the truth. The second, equally important feature is uniformity, that is how even all the ground particles are. An ideal grinder would produce particles of exactly the same size - all of the target size. In reality no grinder achieves this: there is always a certain spread of sizes, called the particle size distribution. The narrower this distribution, the more uniform the grind and the better the coffee. This is why two grinders set to the same coarseness can give completely different coffee - the difference is uniformity. Understanding that not only the coarseness counts but also the evenness of the particles is the starting point for the rest. It is a key but underrated feature of a grinder. We cover grind size more in grind size.

Boulders: too large

The first extreme of the distribution is boulders - particles much larger than the target size. They are too coarse pieces of coffee, forming when the grinder did not grind all the beans evenly. The problem with boulders is that they extract too slowly. A large particle has a small surface-area-to-volume ratio, so water reaches only its outer layer, while the inside stays poorly extracted. The result is under-extraction of this part of the coffee: boulders give off mainly acids, giving a sour, thin, unfinished flavour. It is as if part of the coffee did not have time to brew properly at all. The more boulders in the grind, the more of this sour, under-extracted fraction. Boulders are the extreme of the coarse end of the distribution, a source of unwanted sourness. Understanding that too large particles under-extract is the key to understanding the problem. They are boulders pulling the flavour towards sourness. We cover extraction more in extraction, TDS and the refractometer.

Fines: too small

The second extreme is fines - particles much smaller than the target size. Importantly, fines are largely a physical inevitability, not just a grinder fault. When a coffee bean is crushed, each fracture produces two roughly equal fragments plus a spray of much smaller debris. This extra dust, usually around 30-70 micrometres in size, always forms - it is an inevitable effect of crushing. The problem with fines is the opposite of boulders: they extract too fast. A fine particle has an enormous surface-area-to-volume ratio, so water rapidly draws everything out of it, including the bitter compounds. The result is over-extraction of this part of the coffee: fines give off too much, giving a bitter, astringent flavour. Fines can also clog the filter and slow the flow of water. Understanding that too small particles over-extract completes the picture of the extremes of the distribution. It is dust pulling the flavour towards bitterness.

Why uniformity is key

Now it is clear why the uniformity of the grind is so important. If the grind is uneven, with a lot of boulders and fines at once, a double problem arises: the fine dust over-extracts (bitter), and the large boulders under-extract (sour) - in the same cup. The result is a coffee that is at once bitter and sour, muddy in flavour, without clarity and balance. It is the worst scenario: it cannot be fixed by the coarseness setting alone, because the problem lies in the spread. A uniform grind ensures that each particle extracts at the same rate - they all give off flavour evenly, giving a clean, balanced, sweet coffee. This is why uniformity is the key to good coffee, as important as the coarseness itself. The narrower the particle distribution, the cleaner and more predictable the flavour. Understanding that an uneven grind gives coffee that is both sour and bitter explains why it is worth investing in a good grinder. It is the foundation of clean extraction.

A table: boulders, ideal, fines

Let us gather the particle distribution in one place:

The table shows the heart of it: only particles of the target size extract evenly, and the extremes (boulders and fines) spoil the flavour in two opposite directions. A good grinder minimises both extremes.

Conical versus flat burrs

The kind of burrs in a grinder affects the particle distribution, and thereby the character of the coffee. The two main types are conical burrs and flat burrs. Conical burrs give a bimodal distribution - with two peaks: the target particles plus a clear fines fraction. It is more dust, which paradoxically boosts the body and fullness of the coffee. Flat burrs give a unimodal distribution - more focused around one size, with fewer extremes. This showcases the clarity of the flavour. Neither is objectively better - it is a matter of preference. Conical gives more body and sweetness, flat more brightness and clarity. Professional espresso grinders aim for a very narrow distribution, to ensure an even extraction. Understanding the difference of burrs helps choose a grinder for your own taste. We cover grinders themselves more in burr and blade grinders.

Why a good grinder matters

This knowledge explains why a good grinder is the most important investment in coffee gear - more important even than the machine. A cheap grinder, especially a blade one, produces a chaotic particle distribution: a lot of boulders and fines at once, without control. This dooms the coffee to simultaneous sourness and bitterness, which cannot be fixed. A good burr grinder gives a uniform distribution, minimising the extremes and ensuring an even extraction - the foundation of a clean, balanced coffee. You can have the best beans and the most expensive machine, but if you grind unevenly, the coffee will never be good. This is why baristas repeat: first invest in a grinder, then in the rest. Understanding the role of particle distribution shows why the grinder decides the quality in the cup. It is the heart of good brewing. A good grinder is the key to the potential of every bean. We cover matching the grind to the method more in espresso versus filter.

How to sense it in the cup

The effects of grind distribution are easy to sense once you know what to look for. Coffee from an uneven grind (a lot of boulders and fines) is at once sour and bitter, muddy in flavour, without clarity - the extremes fight each other in the cup. Coffee from a uniform grind is clean, balanced and sweet, with clear, harmonious flavours. If your coffee is strangely sour and bitter at once, despite good beans and settings, the problem is probably an uneven grinder. Notice the clarity: a good distribution gives a clean, clear flavour, a bad one a muddy and chaotic one. It is worth comparing coffee from a cheap blade grinder and a good burr one, to feel how enormous a difference uniformity makes. Over time you will learn to recognise coffee spoiled by a bad grind. It is a skill that points you towards the right gear investments. Uniformity is audible in the clarity of the cup.

The essentials in brief

Let us gather it up. Grinding coffee is not only about coarseness but about uniformity - the distribution of particle sizes. No grinder grinds perfectly evenly: alongside the target particles appear boulders (too large, they under-extract, giving a sour flavour) and fines (dust, too small, physically inevitable, they over-extract, giving a bitter flavour). An uneven grind with a lot of both extremes gives coffee that is at once sour and bitter, without clarity. A good, uniform grinder ensures that all the particles extract evenly, giving a clean, balanced coffee. Conical burrs give more fines and body (a bimodal distribution), flat ones more clarity (unimodal). This is why a good grinder is the most important investment in coffee gear. Now you know why the uniformity of the grind is so key.

Note every coffee in GustoNote - including the grinder and the clarity of flavour you sense. Over time you will start to recognise coffee spoiled by an uneven grind.