To know how much espresso you really have, you need a scale, just as you do when you want to measure your ‘dose’ of coffee grinds. The mass of espresso that comes into the cup is referred to as your beverage weight. If your cafe serves split-shots (where shots are divided into singles by means of a spouted portafilter) then it is best, when measuring beverage weight, to weigh both single shots together. Spouts are not always able to divide shots into exactly equal parts. This is especially true when your recipe calls for a relatively long shot time, e.g., more than 30 seconds. To measure your single-shot beverage weight, you’ll get a reliable average by halving the weight of a double shot.
Baristas work with weight instead of volume because it is a far more accurate way to measure your espresso size. The foam on an espresso (known by its Italian name crema) is made of air bubbles, which are almost weightless. But sometimes these bubbles can be half the volume of an espresso, even though they weigh almost nothing. That’s why most cafes use scales instead of shot glasses to measure the size of an espresso.
Video demonstrating the difference between espresso mass and volume
The Margin of Error for Beverage Weight
The margin of error for beverage weight is established by the recipe designer. It is usually larger than the margin of error for dosing because beverage weights tend to be bigger than doses for most espresso recipes. For example, a common recipe for a double shot would be an 18 ± 0.5-g dose, producing a 36 ± 2-g beverage weight (for a double shot).
Some espresso machines that have preset buttons will be able to deliver more accuracy with beverage weights than others, but most designs can achieve consistency within a ±2-g margin of error.