How fine should I grind for espresso?

The common misconception in espresso preparation is that grind size is a linear variable controlled solely by the user. In reality, coffee grinding produces a bimodal particle size distribution—a mix of larger particles and a significant share of 'fines' (particles under 100 µm). These fines are not merely a byproduct of poor equipment; they are a functional component of the coffee bed. Research confirms that the share of these fines is the primary driver of bed permeability and flow rate, often more so than the median particle size itself. When a producer or farmer delivers a high-density, high-altitude bean, its structural integrity requires a different grinding approach than a lower-density natural process coffee. For the producer, the goal is to preserve the delicate volatile compounds developed during processing; for the barista, the goal is to manage the extraction dynamics of these compounds. If the grind is too fine, the bed becomes impermeable, leading to channeling and uneven extraction. If too coarse, the water bypasses the coffee, resulting in a thin, sour cup. The 'best' grind is therefore the one that optimizes the specific density and solubility of the coffee in your portafilter, rather than a static number on a dial. By focusing on the ratio of fines to larger particles, you move beyond simple 'dialing in' and begin managing the physical architecture of the coffee bed, ensuring that the flavor profile intended by the producer is actually realized in the cup.