“It’s all in the grind” – Ewan McGregor from Black Hawk Down.
Coarse grind = longer brew time
Fine grind = shorter brew time
The coarser you grind your coffee, the less surface area there is for water to come into contact with the bean. Also, Vice Versa, when you grind it finer there is more surface area that comes into contact with water so you can shorten your brew time.
There are solubles in coffee that when water passes through them traps them and brings them into your cup. These solubles are what makes your coffee taste fruity, spicy, nutty etc. They also contain the caffeine and acid in your cup.
The proper grind size is mostly relative to the brew method you are using.
Here’s a basic guide for grind size from coarse to ultra-fine grind.
French Press/ Percolator = Coarse (think kosher salt) 4-5 mins brew
Chemex = Medium-Coarse (think sand) 3-4 mins brew
Other Pour over methods = Medium (think table salt) 230-3 mins brew
Aeropress/ Siphon = Medium-fine (think fine sand) 130-230 mins brew
Moka Pot = Fine grind (you can squish it together) 130-2 mins brew
Espresso = Very Fine 20-40 seconds brew
Turkish = As fine as it gets 10 seconds, 3 times brew
As you can see, the finer you grind the less time you should brew your coffee. This minimizes the bitterness and over-extracted unpleasant flavors from your cup.
All grinders are calibrated differently, so experiment and do what works best for you. Shoot to be in those time ranges for those brewing methods and take notes to see what works best for you!