Grind size is the single most impactful variable you control when brewing coffee. Too fine and your coffee will be bitter and over-extracted. Too coarse and it'll taste sour and watery. The goal is to match your grind to your brew method's contact time and flow rate.
Grind Size by Brew Method
For espresso (25-30 second extraction), you need a very fine grind — almost powdery. Pour-over methods like V60 (2-3 minutes) need medium-fine, like table salt. Aeropress is versatile but usually medium to medium-fine. French Press (4+ minutes of immersion) needs coarse, like sea salt, to avoid sludge and over-extraction.
Choosing the Right Grinder
Invest in a burr grinder — even a hand grinder produces dramatically better results than blade grinders, which create uneven particle sizes. Adjust one click at a time and taste the difference. If your brew is too bitter, go coarser. If it's sour or thin, go finer. Your grinder is the most important piece of coffee equipment you own.