If you want to keep your coffee tasting better for longer, we highly recommend investing in a burr grinder so you can grind your favorite coffee as you need it. You can find a decent quality grinder for around 40 dollars. This investment will improve the quality of your coffee more than any other equipment purchase. However, if you don't have a grinder, we can grind it for you at no extra charge.
Your optimal grind level is largely determined by the brewing method you use. Below are a few of the most common brewing methods and their associated grind levels:
French Press: Coarse Grind
Drip Brewer / K-Cup / Pour Over: Medium Grind
Be aware of the time water is in contact with your coffee grounds. This time dictates your grind level. For instance, in a French Press, water contacts the grounds for 4-5 minutes which gives it the ability to extract a large amount of the contents of the ground. Machines that use K-cups can have a tendency to push water through the grounds too quickly, resulting in a thin cup of coffee. While you can't control the water pressure of those machines, you can choose a slightly finer grind to help slow the water down. On the far end of the spectrum, an espresso typically allows water to be in contact with the grounds for only about a second. To get the same compounds to extract, it requires a much finer grind.
Over-extraction can occur when you use too fine a grind in a brewing method. The water stays in contact with the coffee grounds for a long period of time. This tends to result in bitter coffee.
Under-extraction occurs when you use a grind level that is too coarse for your brewing method. This typically results in coffee that is thin, and sour.