How coarse should cold brew grind be?

Coarse — comparable to a French press, visibly chunky.

Why coarse

Straining. Cold brew is separated after 12 to 18 hours, usually through a paper filter or a mesh. Fines pass through mesh and clog paper, so a fine grind gives you either cloudy coffee or a filter that takes an hour to drain.

Extraction time. The steep is already very long. A fine grind over 16 hours extracts well past the pleasant range and turns woody and harsh.

Sediment. Fines that make it into the bottle keep extracting in the fridge, so the concentrate gets progressively harsher over the week.

How coarse

If it looks like a French press grind, it is right. If in doubt, go coarser — cold brew is far more tolerant of under-extraction than over.

The straining sequence

Strain first through a mesh or a press, then again through a paper filter. Trying to go straight to paper with a full batch clogs it immediately.

If you only have a blade grinder

Cold brew is more forgiving of an uneven grind than any hot method, because the long steep averages things out. It is the best use for a bad grinder.

Ratio reminder

1:8 by weight for a concentrate, 1:15 for ready-to-drink.