Blood Alcohol Concentration (also referred to as Blood Alcohol Content), abbreviated BAC, is the concentration of alcohol in a person’s blood.  One's blood alcohol concentration depends on many factors including the quantity of alcohol consumed, the time frame over which it was consumed, the rate at which it was consumed, the rate at which the person metabolizes alcohol, as well as the person's body size and gender.

So how many drinks can I have and safely drive? 

The important piece of information isn’t how many drinks a person has had, but rather the amount of alcohol the person has consumed.  If the distinction isn’t clear, consider the following pictures…..


I hope these pictures drive the point home, that although each of these gentlemen could honestly say they had only “one drink”, that statement would not provide any useful information in terms of trying to estimate blood alcohol concentration.  We need to know the amount of ethanol in those supersized drinks.  Since it isn’t the number of drinks consumed that’s important, but rather the quantity of ethyl alcohol consumed, we need a uniform system of measurement defining a “standard drink”.