Why grind size does so much
Two things happen when you grind finer. You create more surface area for the water to work on, and you slow down how fast the water drains through. Both of those push extraction up.
So the rule is short enough to remember: sour means grind finer, bitter means grind coarser. Sour is under extraction, you stopped before the sugars came out. Bitter is over extraction, you went too far past them.
Where to start for each method
These are starting points, not rules. Every grinder is calibrated differently, so the useful thing is the relationship between the methods rather than the absolute setting.
| Method | Grind | Why |
|---|---|---|
| Espresso | Very fine | Water is forced through in under 30 seconds, so it needs maximum surface area |
| AeroPress | Medium-fine | Short brew with some pressure, sitting between espresso and pour over |
| Pour over | Medium-fine | Water passes through once, so the grind controls the flow rate |
| Drip machine | Medium | Longer contact than a pour over and less control over the pour |
| French press | Coarse | Four minutes of full immersion, and fines slip through the mesh |
| Cold brew | Coarse | Twelve hours or more, so a fine grind would be badly over extracted |
The pattern underneath the table is simply contact time. The longer the water sits with the coffee, the coarser you go. Espresso has thirty seconds so it needs everything fine. Cold brew has twelve hours so it needs everything coarse.
Move slowly
When you are adjusting, go two clicks on a hand grinder or one or two steps on an electric one, and then taste. Jumping ten settings will overshoot and you will learn nothing about your own grinder except that it can make bad coffee in two directions.
Burrs versus blades
A blade grinder chops coffee into random sizes, so you end up with dust and boulders in the same batch, and the dust over extracts while the boulders under extract in the same brew. A burr grinder crushes beans to a consistent size between two surfaces.
A cheap burr grinder will do more for your coffee than an expensive brewer paired with a blade grinder. If you are buying one thing, buy this.
Try this
Brew the same coffee twice tomorrow and change nothing but the grind. One noticeably finer, one noticeably coarser, everything else identical. Taste them next to each other. That single experiment calibrates your own grinder in a way no chart can do for you.