Grind by roast, not by brewer

Every brew recipe on this site gives you a grind setting. Those numbers assume a medium roast, because that's most of what I drink, and they will be wrong for a very dark bag or a very light one.

That's not a flaw in the recipe. It's a thing most grind advice leaves out: the brewer tells you roughly where to start, and the roast tells you which way to move from there.

Why roast changes the grind

Roasting makes coffee more soluble. The longer and hotter the roast, the more the cell structure of the bean breaks down, and the more readily it gives up what's in it to hot water.

So the same grind, in the same brewer, for the same time, extracts more from a dark roast than from a light one. Past a point, what you're extracting is the bitter end of the range.

Dark roasts are also more brittle. They shatter rather than cut, so a given setting produces more fines than it would with a light bag, and fines extract fastest of all. Two effects pointing the same way.

The adjustment

Dark roast: a tick or two coarser. Fewer fines, slower extraction, less of the bitter end. This is the one that matters most, because over-extracted dark coffee is the single most common bad cup people make at home.

Light roast: a tick or two finer. Denser bean, less soluble, and it needs the extra surface area or you get something thin and sour that people mistake for "bright".

Medium: the number in the recipe. That's what it was written for.

A tick on my Fellow Ode 2 is about 0.2. On a hand grinder it's a click or two. The size of the step matters less than the direction.

How you know you got it wrong

Bitter, harsh, drying: over-extracted. Go coarser. If it's a dark roast, this is the likely answer before you touch anything else.

Sour, thin, tastes like it stopped early: under-extracted. Go finer. With a light roast, this is the likely answer.

Both at once, which is confusing: usually too many fines. Coarser, and check the grinder.

The useful habit is to change one thing and taste. The whole reason for weighing coffee is that it lets you do that honestly, and a grind change with everything else held still is the clearest experiment in brewing.

What this doesn't mean

It doesn't mean dark roasts are worse. A good dark roast ground correctly is excellent, and the reason people think otherwise is that most of them have only had it over-extracted.

And it doesn't mean the recipe numbers are unreliable. They're a starting point, written for the bag I had in front of me. Yours will be somewhere nearby, and finding out where is the point.