This heavy-weight shot glass is marked at 1 fluid ounce (1 fl.oz.) When you make your espresso you need to adjust your coffee machine so that you obtain 1 fl.oz. of espresso (per cup) in approximately 25 seconds; make the grind more coarse if it takes too long to fill up to the line, or make it finer if it takes less than this time to reach the line. Professional baristas use this type of shot glass to calibrate their coffee machine, and some prefer to make every espresso shot into a shot glass so they can monitor their espresso throughout the day.