What grind size for an AeroPress?

Medium-fine is the standard starting point: finer than a V60, noticeably coarser than espresso. Somewhere around fine table salt.

Why the answer is vague

The AeroPress is the most recipe-flexible brewer in common use, and grind size is tied to steep time. Both are levers, and they trade off:

• Fine grind, short steep — 30 to 60 seconds. Fast, punchy, concentrated.
• Medium grind, medium steep — 1:30 to 2:00. The standard.
• Coarse grind, long steep — 3 to 5 minutes. Gentler, closer to a French press.

All three work. What matters is that they match.

Adjusting

If the press is very hard work, go coarser — you should be able to press in twenty to thirty seconds without leaning on it.

If the coffee tastes weak or sour, go finer or steep longer. If bitter, coarser or shorter.

Why it forgives a modest grinder

Immersion averages out an uneven grind over time in a way pour-over cannot. This is the brewer that makes a cheap grinder acceptable, which is exactly why it is the right first brewer.

Metal filters

A metal filter needs a coarser grind than paper — fines pass straight through and make the cup silty.