Easy rule: The longer the brew time, the coarser the grind.
Taste sour? Make it finer
Taste bitter? Make it courser

It is often stated that the most important things you can do to make better coffee is to grind fresh and use a scale. Consistency is key in this theory. You want to be able to have the same great cup every morning (or afternoon) and enjoy your morning ritual. On the flipside, you want to be sure you can avoid making that face puckering brew you just struggled through.

”What is the ideal grind size?” is probably one of the most frequent questions I get. At best, we see pictures and diagrams on grocery store grinders, but more often we just see a number or dots approximating a presumed size. But what does it mean? Unfortunately, there isn’t much consistency between grinders. Each manufacturer has their own specifications as to what a “2” means, and this doesn’t even account for the purpose of the grinder. The same manufacturer might have a grinder for espresso and one for pour over, both with a number “2” setting, and yet, they are completely different grind sizes. It can be confusing and cause one to simply give up.

Fortunately, there is a generic rule one can follow. It may not fit all situations, but given that most people often only brew coffee using one method, one can use this rule and adopt the “set it and forget it” mantra. The rule is this: the longer the brew time, the coarser the grind. Brew methods such as the French press often follow a longer brew time in which all of the coffee is in contact with all of the water (immersion). The brew we consume is simply the result of extraction. Hot water extracting soluble material from the beans (some taste good, some taste bad). By adjusting the grind size, you are decreasing or increasing the surface area of the coffee bean fragments in which the water has to interact and extract. A finer grind means more surface area, which means a greater potential for extraction and thus the requirement for less time to extract. Now, there are other factors that can impact extraction, such as water temperature, roast level, or pressure. However, as I often say, “coffee is as complicated as you want it to be,” so let’s just make this one simple.