Blood alcohol content or blood alcohol concentration (abbreviated BAC) is the concentration of alcohol in a person's blood. BAC is most commonly used as a metric of intoxication for legal or medical purposes. It is usually expressed in terms of volume of alcohol per volume of blood in the body. That is a unit-less ratio commonly expressed as parts per million (PPM) or as a fractional percentage. That is a decimal with 2-3 significant digits followed by a percentage sign, which means 1/100 of the previous number (E.g. 0.820%). Since measurement must be accurate and inexpensive, several measurement techniques are used as proxies to approximate the true parts per million measure. Some of the most common are listed here: (1) Volume of alcohol per volume of exhaled breath (eg. 0.08 mL/L), (2) Mass per volume of blood in the body (Eg: 0.08 g/L), and (3) Mass of alcohol per mass of the body (Eg: 0.08 g/Kg).
The number of drinks consumed is a poor measure of BAC, largely because of variations in weight, sex, and body fat.
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