A shot of espresso that is equal to 1.5 ounces contains approximately 77 milligrams of caffeine; if the shot is a single ounce, it has about 64 milligrams of caffeine. Espresso serving sizes vary greatly at individual coffee shops. It is brewed by forcing a little bit of almost-boiling water through finely ground coffee grains.
It is safe for the average adult to consume 300 to 400 milligrams of caffeine on a daily basis; this is equal to approximately 5.2 espresso shots. Children 12 and under should not consume caffeine on a daily basis, and kids between the ages of 13 and 18 should not have more than 100 milligrams of caffeine in a single day.