Blood alcohol level is expressed as Gram Percent. That means grams per 100 grams of whole blood. In most states the legal limit for blood alcohol in drivers is 0.08 Gm% (years ago it was higher, 0.10). In many European countries the limit is far stricter, on the order of 0.06 Gm%.
A normal person will achieve a blood alcohol level of roughly 0.02 Gm% after one drink (1 shot of hard liquor, or 1 beer or glass of wine).
A chronic alcoholic who drinks daily will develop a tolerance to alcohol. For a given blood level she may seem unaffected, whereas a naive drinker at the same level would appear to be grossly intoxicated.
A normal person with a healthy liver is able to metabolize (burn off) approximately 0.02 Gm% per hour. A chronic alcoholic with cirrhosis of the liver may metabolize the alcohol much slower, and is in danger of intoxication after drinking an amount of alcohol that a healthy person could ordinarily handle.