Grind size, explained by what you are brewing into

Every guide tells you that espresso wants a fine grind and a French press wants a coarse one. Very few explain why, which means the advice does not transfer the first time you try something new.

Here is the rule underneath it: grind size is set by how long the water and the coffee are in contact.

The trade

Water pulls flavour out of coffee at a rate that depends on how much surface area it has to work on. Grinding finer creates enormously more surface area, so extraction happens faster.

That gives you one lever and two ways to be wrong:

• Under-extracted coffee tastes sour, thin and salty. Not enough came out.
• Over-extracted coffee tastes bitter, harsh and drying. Too much came out.

Long contact needs a coarse grind or it over-extracts. Short contact needs a fine grind or it under-extracts. That is the entire relationship, and every method is a point on it.

The methods, ordered by contact time

• Espresso, 28 seconds. The finest grind there is. Powder, almost.
• Moka pot, four and a half minutes but under pressure. Fine, though coarser than espresso, and never tamped.
• South Indian filter, twenty minutes by gravity through a packed bed. Fine, and packed rather than tamped.
• AeroPress, two and a half minutes with pressure at the end. Medium-fine.
• Pour-over and Chemex, three to four minutes. Medium, like table salt.
• Channi, four minutes. Medium-coarse, and limited by what your strainer holds back.
• French press, four minutes through a metal mesh. Coarse, like sea salt.
• Cold brew, sixteen hours. The coarsest of all.

Read down that list and the pattern is visible: as contact time goes up, the grind goes coarser, with the exception of the pressure methods, which get their speed from force instead of fineness.

Diagnosing a cup by taste

This is more useful than any chart:

• Sour, sharp, watery? Under-extracted. Grind finer, or brew hotter or longer.
• Bitter, dry, ashy? Over-extracted. Grind coarser, or brew cooler or shorter.
• Weak but not sour? Not an extraction problem. Use more coffee.

The third one catches people out constantly. Weak coffee is usually a dose problem, and grinding finer to fix it produces bitter weak coffee instead.

Consistency beats precision

An even grind matters more than the exact setting. Water flows through the path of least resistance, so a mix of powder and boulders gives you over-extraction from the fines and under-extraction from the chunks, in the same cup, at the same time. The result tastes simultaneously bitter and sour and cannot be fixed by adjusting anything.

This is the whole argument for a burr grinder over a blade grinder. A blade grinder chops at random. A burr grinder produces particles of a similar size.

Grind when you brew

Coffee goes stale within minutes of grinding, because grinding is what exposes all that surface area to air. Whole beans stay good for weeks; ground coffee is noticeably flatter the next day.

If you buy whole beans, the recipe for your coffee on any of the brewing guides states the grind setting alongside the dose and the temperature, so you are not translating "medium-fine" into a number on your own.