In any method of brewing, the grind size is very important and more so when it comes to espresso coffee. Grind size controls how much extraction happens when water moves through ground coffee. The finer the grind size, the more extraction happens and vice versa. Ground coffee used for espresso needs to be of fine or extra-fine grind size.
Why? It all comes down to what can be called espresso. To be considered espresso, ground coffee needs to be brewed under at least 9 bars of pressure in a short period of time to give you a rich, concentrated shot of coffee with a creamy layer. In order to get the correct concentration and to make sure that the ground coffee doesn’t over- or under-extract, you need to use a fine grind that will hold the water for the ideal time. Fine or extra-fine grind size is comparable to table salt, so keep this in mind when buying at your local store or café. If the water is taking too long to pass through the ground coffee, then you need to try something coarser and if it’s passing through too fast, you need to make your grind size finer.
Unfortunately, it’s not a good idea to regrind your coffee to make it finer. If you regrind ground coffee, it can clog your grinder and make it a hassle to clean. It’s better to buy fresh, fine ground coffee for your espresso rather than regrind any ground coffee you have.