Blood Alcohol Content and the Legal Drinking Limit

Vial of Blood
Blood and Urine Test Measure BAC. © Getty Images

Blood-alcohol content refers to the amount of alcohol contained in a person's blood. It is expressed in terms of weight per unit of volume.

Blood alcohol content is the amount of alcohol present in a 100 milliliter (mL) volume of blood. For example, 80 mg is 0.08 grams, 0.08 grams of alcohol in 100 mLs is written as 0.08%. In other words, 80 mg% is equal to 0.08% which is equal to 80 mg/dL (deciliter; 100 mLs).

This value can also be described as 0.08 BAC. All of these methods of expressing blood alcohol concentration are in use in various countries.

In the United States, a blood-alcohol content of 0.1 (0.1% or one tenth of one percent) means that there are 0.10 g of alcohol for every dL of blood in the person's body at the time of the test.

Blood-alcohol content is used for legal and medical purposes to indicate a person's level of intoxication. In all 50 states and the District of Columbia, the legal limit for driving under the influence of alcohol is .08.

But, impairment begins before a person reaches the legal limit of .08, many studies have found. When alcohol is consumed and absorbed into the bloodstream, it travels directly to the brain, affecting many cognitive functions.

Legal Levels of BAC and Impairment

Research has found that with a blood-alcohol content as low as .02, people can exhibit some loss of judgment, relaxation, slight body warmth and altered moods.

At a blood-alcohol level of .05, the person may begin to exhibit loss of small-muscle control - such as being able to focus the eyes - and lowered alertness.

At the legal level of .08, the person will usually exhibit:

  • Poor muscle coordination
  • Loss of balance
  • Slower reaction time
  • Slurred speech
  • Loss of vision and hearing
  • Difficulty in detecting danger
  • Impaired judgment, self-control
  • Impaired reasoning and memory

Information processing, decision making, and hand-eye coordination are impaired in some people with blood-alcohol content levels as low as .04.

All of these impairments result in an increased risk of injuries in general, and particularly those related to the operation of a motor vehicle.

Driving skills can be impaired long before someone reaches the legal limit, but at .08 the risk of having a vehicle crash increases dramatically.

Also Known As: BAC, blood alcohol level, blood alcohol concentration and blood ethanol concentration.

Examples: When arrested his blood alcohol content was .17.


National Highway Traffic Safety Administration. "The ABCs of BAC." A Guide to Understanding Blood Alcohol Concentration and Alcohol Impairment 

The University of North Carolina Highway Safety Research Center. "Blood Alcohol Concentration (BAC)." Alcohol Studies