Blood alcohol concentration (BAC) is measured by blowing into a breath alcohol device. The results are given as a number which shows the level of alcohol in the blood. The legal limit for being considered impaired while driving is .08.
However, US Department of Transportation regulations consider a .04 a positive alcohol test requiring removal from driving or other safety-sensitive tasks. A .02 under DOT regulations can require removal from tasks for a certain period of time.
Blood alcohol tests show current levels of impairment or intoxication not past use.
Blood / Breath Alcohol Concentration (BAC) Calculators
There are calculators available to compute your estimated blood/breath alcohol concentration based on the number of drinks you have consumed and how quickly you drank them, your weight, and your gender.
Drug and Alcohol Tests