If your coffee tastes wrong and you only change one thing, change the grind. It moves the cup further than water temperature, brand of filter or the price of your kettle.
What grind actually does
Grinding increases the surface area the water can reach. Finer grounds extract faster because there is more surface and shorter distance to the centre of each particle. Coarser grounds extract slower.
Every brew method gives the water a different amount of contact time, so each one needs a different grind to land in the same place.
Reading your cup
Sour, thin, watery, finishes fast. Under-extracted. The water didn't pull enough out. Grind finer.
Bitter, harsh, drying, lingers unpleasantly. Over-extracted. The water pulled too much. Grind coarser.
People often reach for more coffee when the cup tastes weak, but weak and sour usually means under-extracted, not under-dosed. Try the grind first.
The starting points
Coarse — French press, cold brew, percolator. Looks like sea salt. Long contact time, so the particles need to be big.
Standard — drip machines, pour over, Aeropress. Looks like table salt. The all-purpose setting.
Espresso — espresso machines, moka pots. Looks like fine sand. Water is forced through in under 30 seconds, so it needs maximum surface area.
Grind fresh where you can
Ground coffee loses its aromatics within days because so much surface is exposed to oxygen. Whole bean keeps for weeks. If you can grind at home, do — and if you can't, buy smaller bags more often rather than one big one.
