Blood alcohol content is expressed as a percentage of alcohol in the blood—as an example, if a person has a BAC of 0.10, then one-tenth of one percent of that person’s blood, by volume, would be alcohol. All states except Utah have set 0.08% BAC as the legal limit for DUI. (Utah very recently lowered their threshold to 0.05%, a change which went into effect on December 30th). For commercial drivers, the limit is 0.04%, and in most states, for those under 21, there is a zero-tolerance limit, meaning any amount of alcohol could result in a DUI arrest.
The exception to the 0.08% BAC is in the state of Arizona, where you can be charged with DUI with any level of alcohol in the body if your driving exhibits any degree of impairment. This means that while the presumption of impairment is 0.08% in Arizona, showing signs of impairment in your driving ability could still get you arrested and convicted, even if your BAC is less than 0.08%.