Foundational

Grind size explained

How grind size affects extraction, and which grind range suits which brewing method.

Why grind size matters

Grind size controls how much surface area of the coffee is exposed to water, and how easily water can flow through the coffee bed. Finer grinds extract faster and create more resistance to water flow; coarser grinds extract more slowly and let water pass through more easily.

Matching grind to method

Espresso uses a fine grind because contact time is only around 25-30 seconds under pressure. Pour-over uses a medium grind for a multi-minute percolation brew. French press and cold brew use a coarse grind because they are full-immersion methods with long contact times.

Diagnosing problems from flow rate

If a brew runs faster than expected for its method, the grind is often too coarse (or channelling is occurring); if it runs slower than expected, the grind is often too fine. Adjust grind size before changing other variables.

Uneven grind: fines and boulders

Even at a single grind setting, a grinder produces a spread of particle sizes rather than one uniform size. The extremes of that spread are commonly called fines (very small particles, close to dust) and boulders (oversized, under-crushed fragments). Fines over-extract quickly because of their large surface area relative to their volume, and can also clog filters or paper, slowing the brew and adding bitterness or a muddy texture. Boulders under-extract because water passes them with little contact, contributing sourness and weak flavour to the same cup. A grinder with a wide, inconsistent particle-size distribution can therefore produce both over- and under-extracted flavours in the same brew at once, which is why grind consistency (not just average grind size) is a separate variable from the grind-size setting itself. Burr grinders — see choosing a grinder — produce a narrower distribution than blade grinders and are the main lever for reducing fines and boulders.

Static, retention and grind heat

Static causes freshly-ground coffee to cling to the grinder chute, hopper walls or portafilter instead of falling cleanly, mainly from friction between very dry grounds and the equipment; a few drops of water on the beans before grinding (sometimes called the Ross Droplet Technique) or simply letting very fresh, gassy beans rest a day or two can reduce it, though neither eliminates it completely on every grinder. Retention is grounds left behind inside the grinder's chute or burr chamber after grinding rather than reaching the cup — a small amount is normal on most grinders, but a build-up of stale, retained grounds can mix into a later, fresher grind and taint its flavour, which is part of why regular cleaning (see choosing a grinder) matters. Grinding also generates a small amount of heat from friction; for the short grind times used at home, this is generally considered a minor factor compared with grind size, dose and freshness, and is rarely worth addressing beyond not leaving beans grinding for far longer than necessary.

When to change grind versus dose

Grind size and dose both affect strength and flow, but they are not interchangeable: grind size mainly changes how fast water flows through the coffee bed and how evenly it extracts, while dose mainly changes how much coffee is in the cup relative to water. As a general starting point, adjust grind first when a brew's flow rate or timing is off for its method (runs too fast or too slow — see diagnosing problems from flow rate, above); adjust dose or ratio first when the flow rate and timing already look right for the method but the resulting strength itself is what needs to change. Changing both at once makes it hard to tell which change caused which effect.

Sources

SCA brewing and espresso standards — Specialty Coffee Association (SCA)