Blood alcohol concentration (BAC) is a way of measuring how much alcohol is in the blood. This, rather than the exact amount of alcohol the person has consumed, will determine the effects the alcohol will have on the person's brain, body and behavior.
The amount of alcohol in the bloodstream is measured in mg of alcohol per 100ml of blood. It is usually shown as a decimal, such as 0.08 or 0.15. It is sometimes shown as mg%, such as 80 mg% or 150 mg%.
Your BAC can be assessed accurately from a breath test or breathalyzer. Many jurisdictions have laws which limit people's BAC while driving.