What grind size for a French press?

Coarse. Think coarse sea salt or fresh breadcrumbs — visibly chunky, with individual particles you can pick out.

This is the most-ignored piece of advice in coffee, and the consequences are exactly what people complain about.

Why coarse

A French press separates coffee from water with a metal mesh. Metal cannot hold back fine particles the way paper can, so every fine your grinder produces ends up in the cup. That is the sediment, and it keeps extracting after you pour, which is why the second half of a cafetière tastes harsher than the first.

Grinding coarse gives you fewer fines to begin with. It also slows extraction, which is appropriate: the coffee is steeping for four minutes rather than passing through in thirty seconds.

The grinder matters more here than people think

Grind coarse on a blade grinder and you get a mixture of boulders and dust, because a blade chops randomly rather than cutting to a size. The dust makes sludge, the boulders contribute nothing, and the cup is both bitter and weak.

A burr grinder set coarse produces a genuinely uniform grind. That single change transforms French press coffee more than any technique.

Two habits that help

Break the crust and skim at four minutes rather than stirring the foam back in — a lot of the bitterness sits there.

Decant immediately into a jug or cups. Coffee left sitting on the bed keeps extracting, and the last cup is noticeably harsher than the first.

Press gently and stop before the plunger reaches the bottom. Forcing it drives fines through the mesh.