Blood Alcohol Concentration (BAC)Definition - What does Blood Alcohol Concentration (BAC) mean? Blood alcohol concentration / blood alcohol content (BAC) refers to the amount of alcohol content in an individual's blood. It is the most commonly used metric for measuring a driver's level of intoxication for legal purposes like driving under the influence (DUI/DWI). Blood alcohol concentration is measured in weight per unit volume. It is then converted into a percentage. Justipedia explains Blood Alcohol Concentration (BAC) Driving under the influence (DUI) is a major cause of traffic accidents in the United States. Due to the effect that alcohol consumption has on cognitive functioning, it is illegal to drive while being intoxicated. BAC is a metric measurement used for gauge the amount of alcohol that an individual has consumed. BAC is determined by measuring the weight of alcohol present per unit volume of blood, and then converting it into a percentage. In the United States, it’s illegal to operate a motor vehicle with a BAC of 0.08% or higher.