Coffee Particle-Size Distribution
Particle-size distribution (PSD) describes how ground coffee is divided among different particle sizes. A grinder does not produce one perfectly uniform diameter; every dose contains a distribution that can include a main target population, smaller fines, and larger particles often called boulders.
Why PSD matters for brewing
Particle size affects both extraction rate and water flow. Smaller particles have more exposed surface area and tend to extract quickly. In espresso, they also contribute substantial hydraulic resistance. Larger particles extract more slowly and can contribute less soluble material during the same brew.
Fines are not automatically bad
Some fines are produced by every coffee grinder, and they can be useful. Traditional espresso depends on a distribution capable of creating enough puck resistance for controlled flow. In filter brewing, however, excessive fines can migrate into paper pores or the bottom of the coffee bed and slow drawdown.
Narrower is not always better
A very narrow distribution can increase flavor separation and clarity, but that does not make it universally superior. Some espresso drinkers prefer the body and integrated flavor associated with burrs that produce a broader distribution. Grinder design should be matched to the intended cup profile.
What changes particle distribution?
- Burr geometry and manufacturing
- Burr alignment and condition
- Grinder RPM and feed behavior
- Roast level and bean brittleness
- Grind setting
- Regrinding or retention inside the chamber
Measuring PSD has limitations
Sieves, laser diffraction, imaging, and other methods can report different values because each measures particles differently. Sample preparation also matters. A chart should therefore be interpreted in the context of the measurement method rather than treated as an absolute flavor score.
For broader burr-design context, read our conical vs. flat burr comparison. Browse home coffee grinders or explore more terms in the coffee glossary.
