How is Blood Alcohol Concentration Determined?

The amount of alcohol in a person's body is measured by the weight of the alcohol in a certain volume of blood. This is called the blood alcohol concentration, or "BAC." A driver's BAC can be measured by testing the blood, breath, urine or saliva. Breath testing is the primary method of testing used by law enforcement agencies. 

BAC is highly related to the amount of alcohol consumed over time. However, BAC is also influenced by other factors and the complex relationship makes it difficult for people to easily estimate BACs. For example, what is the BAC of a 125 pound woman, who is not an experienced drinker, who has four beers over two hours? Use the following form to find the answer:

Blood-Alcohol Concentration Calculator
(U.S. pounds)
Drinks Consumed:
(12 oz beer or equivalent)
Timeframe Consumed:


It should be understood that what results is an estimate because the calculation is made on average values, for example, the amount of water in the body and in the blood, the average metabolism rate for a population, etc. The value estimated will certainly be correct for most individuals sharing characteristics placed into the computation, but may be greater or lesser depending on individual factors of which we do not have knowledge.

The estimate of BAC should not be used by individuals to decide whether or not to drive after drinking -- impairment can result from any amount of alcohol. However, it does provide the best available approximation of the number of drinks it takes for individuals to reach specific BACs.

Breath testing equipment is evaluated for precision and accuracy by the National Highway Traffic Safety Administration (NHTSA). Test instruments approved by NHTSA as conforming to specifications are accurate within plus or minus .005 of the true BAC value.

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