How does grind size affect extraction time?

The relationship between grind size and extraction time is governed by the principles of surface area and flow rate. Imagine trying to wash a pile of sand versus a pile of large pebbles; the water flows easily through the pebbles, but gets "held up" by the dense, fine sand. Coffee particles work in exactly the same way.

When you grind your coffee finely, the particles pack closely together. In methods like espresso, this creates a dense "puck" that creates significant resistance. Because the water has to work hard to push through this barrier, the contact time is extended. This is desirable in espresso because the high pressure forces water to extract concentrated sugars and oils from those tiny particles very efficiently. However, if the grind is too fine, the water may stop moving altogether, leading to a blocked machine or an over-extracted, bitter cup.

Conversely, a coarse grind creates more space between particles. Water passes through this bed with little resistance. If you used a coarse grind for espresso, the water would rush through in seconds, resulting in a thin, sour, and watery shot because it didn't have enough time to interact with the coffee. However, in a French press, this coarse grind is perfect because you are allowing the coffee to steep for several minutes; the larger particles prevent the coffee from becoming bitter during that long immersion time.

Practically speaking, if you find your coffee is taking too long to brew or is becoming bitter, it is a signal that your grind is likely too fine for your method. If the brew is too fast and tastes weak or sour, your grind is likely too coarse. Mastering the relationship between these two variables allows you to "dial in" any coffee, ensuring that you are consistently extracting the best possible flavor regardless of whether you are using a V60, a Chemex, or a standard espresso machine.