Grind Size and Particle Distribution

Water can only dissolve what it touches. Grinding turns a bean with a few square centimetres of surface into thousands of particles with hundreds of times that area, and the size of those particles is the main way you control extraction rate.

Why finer extracts more

Extraction happens in two stages: the surface of a particle is washed almost instantly, then water has to diffuse into the particle and dissolved coffee has to diffuse back out. Diffusion time scales with the square of distance, so a particle half the size extracts in a quarter of the time. Finer grind means:

• more surface area per gram — faster initial extraction;
• shorter diffusion paths — the inside of each particle gets extracted too;
• in a percolation brew, slower flow — longer contact time on top of the above.

All three push extraction up. That is why "grind finer" is the fix for sour coffee and "grind coarser" is the fix for bitter coffee.

Typical settings by method

Method	Grind	Approx. particle size
Turkish	powder	< 200 µm
Espresso	very fine	200–400 µm
Moka pot	fine	400–600 µm
AeroPress	fine-medium	500–800 µm
Pour over	medium	600–1000 µm
French press / cold brew	coarse	1000–1400 µm

Numbers are indicative — grinders differ, and roast level, freshness and bean density all shift the sweet spot. The calculator uses a 1–10 scale (1 = finest, 10 = coarsest) as a common language across grinders.

Fines and boulders

No grinder produces a single particle size. Every grind is a distribution, and the shape of that distribution matters as much as the average.

• Fines (particles under ~100 µm) extract almost instantly and then keep giving up bitter compounds. They also clog filters and slow drawdown. Some fines are essential for body, but excessive fines are what make a cup taste harsh and under-extracted at the same time — the fines over-extract while the boulders under-extract.
• Boulders (oversized particles) contribute mostly their surface and stay sour and under-extracted inside.

A tighter distribution lets you push extraction higher before bitterness shows up, which is why good grinders let you brew at 22–23 % EY while cheap ones top out around 19–20 %.

Burrs, blades and alignment

• Blade grinders chop randomly and produce a wide distribution. Usable for French press; frustrating for anything else.
• Conical burrs crush along a cone. Slightly wider distribution with more fines; produce body and forgiving cups, popular for espresso.
• Flat burrs shear between two parallel discs. Tighter distribution, higher clarity, more sensitive to grind adjustments.
• Burr alignment matters: even a tenth of a millimetre of wobble widens the distribution noticeably.

Practical rules

1. Change grind in small steps and change one thing at a time.
2. Dark roasts are more brittle and produce more fines — grind a step coarser than you would for a light roast.
3. Denser, high-grown beans need a finer setting than soft, low-grown beans to hit the same extraction.
4. A pour over that stalls or drains in under two minutes is telling you about grind before it tells you about anything else.
5. Purge the grinder when switching coffees; retained grounds from the last setting will muddy your first cup.