Blood Alcohol Content (BAC) is the level of alcohol (understood as ethanol) circulating in the bloodstream. For example, a BAC level of 0.20% means that a person’s bloodstream contains 0.20g/dL (milligrams per deciliter) of alcohol for every 100 ml of blood.
Alcohol is processed by the liver with about one standard drink per hour. One standard drink or “one dose” of alcohol depends on the alcoholic beverage: 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of liquor are equivalent and measured as one standard drink of alcohol. When a person drinks faster than the liver can process, the effects of intoxication will become prevalent.
The effects of intoxication evolve when the BAC goes up and can range from slurred speech, loss of balance and coordination, mood changes to vomiting and blackouts, to comas, or death.
After a certain level of BAC is reached, it becomes illegal to drive a motor vehicle, and in some states also a bicycle. In most states, it’s illegal and constitutes a criminal offense to drive with a BAC of 0.08% or higher (see: Driving Under the Influence (DUI)). The BAC can differ in each person depending on factors such as weight and other health factors. For example, a person with a body weight of 120 lb. will typically reach a BAC of 0.08% after approximately two to three drinks, and a person with a bodyweight of 180 lb. will reach a BAC of 0.08% after approximately three to four drinks.
See an example of a BAC calculator here.
Police officers may use blood alcohol tests to measure BAC.
[Last updated in March of 2022 by the Wex Definitions Team]