Traditional Espresso aka Normale
Italian-style espresso, with a high TDS and persistent crema, only came into existence after the invention of the modern espresso machine in the 1940s. Depending on what espresso machine you use and what brew basket, it is possible to use doses ranging from around 5g up to as high as 27g. Exactly how much is considered appropriate is subject to debate.
A traditional Italian espresso will tend to be brewed with lower doses than what is common in specialty coffee shops. One very strict definition can be seen here from the International Organisation for Standardisation (ISO). The ISO Standard 45011 gives Italian espresso as:
Dose 7g =+/- 0.5g
From the traditional vernacular surrounding espresso, the term ‘shot’ suggests a bartender’s shot which is traditionally one fluid ounce. But even ounces vary in volume; a US fluid ounce is 30ml and an imperial ounce is approximately 28.4ml. Here is an excerpt from the most recent publication from the World Barista Championships which uses the ‘ounce’ as part of the definition. This definition is not concerned with what dose is to be used.
Rule 2.2.1 from page 6 of the World Barista Championships Rules and Regulations
In contrast, Andrea Illy and Rinantonia Viani are specific about doses in their book, Espresso Coffee, The Science of Quality: “Ground coffee portion 6.5g +- 1.5g” (Chapter 1.7 Definition of Espresso pg.19)
Concerning volumes, they comment: “Everyone in Italy has a clear mental picture of a cup of espresso: a small heavy china cup with a capacity just over 50ml, half full with a dark brew topped by a thick layer of reddish brown foam of tiny bubbles known by the Italian term ‘crema’.” (Chapter 1.7 Definition of Espresso)
Andy Schecter’s Brewing Ratio for Espresso Coffee
Andy Schecter’s Coffee Ratios for Espresso Coffee