Blood alcohol content, or BAC, is the weight of alcohol present in a certain volume of your blood, measured in grams per deciliter. A BAC of 0.08%, the legal driving limit in most of the United States, means there are 0.08 grams of alcohol in every 100 milliliters of your blood. It’s the standard measurement used by law enforcement, hospitals, and researchers to quantify how much alcohol is affecting your body at any given moment.
What Counts as One Drink
BAC is easier to understand once you know what a “standard drink” actually means. In the United States, one standard drink contains 0.6 ounces (14 grams) of pure alcohol. That works out to:
- 12 ounces of beer at 5% alcohol
- 8 ounces of malt liquor at 7% alcohol
- 5 ounces of wine at 12% alcohol
- 1.5 ounces of distilled spirits (gin, vodka, whiskey) at 40% alcohol
A pint of craft IPA at 7% or a generous pour of wine at a restaurant can easily contain 1.5 to 2 standard drinks. This matters because most people estimate their intake by counting glasses or bottles, not by tracking actual alcohol volume.
How Your Body Processes Alcohol
After you take a drink, alcohol is absorbed through the lining of your stomach and small intestine into the bloodstream. Your BAC rises as alcohol enters the blood faster than your liver can break it down. For a person weighing about 155 pounds (70 kg), the liver removes roughly 7 grams of alcohol per hour, which is approximately one standard drink per hour.
That rate isn’t perfectly constant. At higher concentrations, the liver’s processing speed shifts slightly depending on how saturated its enzymes are. But for practical purposes, the “one drink per hour” estimate is a useful baseline. If you consume three drinks in an hour, your liver can only handle one of them in that time, and the remaining alcohol stays circulating in your blood.
Nothing speeds this process up. Coffee, cold showers, and food eaten after drinking do not lower your BAC faster. Time is the only thing that clears alcohol from your system.
Why the Same Amount of Alcohol Hits People Differently
Two people can drink the same amount and end up with very different BAC levels. The biggest factors are body size, body composition, and biological sex. Women typically reach higher BAC levels than men after the same number of drinks, even when body weight is similar. This happens because women generally carry a higher proportion of body fat and less body water. Since alcohol dissolves in water, having less water in the body means the same amount of alcohol becomes more concentrated.
Other variables matter too. Drinking on an empty stomach allows alcohol to reach the small intestine faster, where absorption is most efficient. Carbonated drinks can speed absorption. Medications that affect the liver can alter how quickly alcohol is broken down. Genetic differences in liver enzymes also play a role, which is why some people feel the effects of a single drink more than others.
How BAC Affects Your Brain and Body
Impairment starts well below the legal limit. Even at a BAC of 0.02%, roughly one drink for many people, subtle changes in judgment and relaxation begin. You might feel slightly warmer and more sociable, but reaction time is already starting to slow.
At 0.05%, the effects become more noticeable. Coordination declines, inhibitions lower, and your ability to track moving objects weakens. Driving studies show that speed variability, the tendency to unintentionally speed up and slow down, increases significantly at this level. Some countries set their legal driving limit at 0.05% for this reason.
At 0.08%, the legal limit across most U.S. states, impairment is clear. Lane position becomes erratic, reaction time drops substantially, and the ability to process multiple pieces of information at once is compromised. Research on driving simulators confirms that at 0.08%, drivers drift out of their lanes significantly more than at lower BAC levels or when sober.
Above 0.15%, the risk of falling, vomiting, and blackouts rises sharply. At very high levels, alcohol begins to suppress the brain regions that control breathing, heart rate, and body temperature. This is alcohol overdose, and it can cause loss of consciousness and death. There is no single BAC number that marks the line between survival and fatality, because tolerance, body size, and overall health all play a role. But the danger zone starts when basic life-support functions are disrupted.
How BAC Is Tested
The two most common methods are breath testing and blood testing. Breath tests (breathalyzers) work by measuring the amount of alcohol in the air you exhale, then converting that to an estimated blood concentration. Blood tests measure alcohol directly from a drawn blood sample and are considered the more accurate of the two.
A large comparison study of over 400 paired breath and blood results found that breathalyzers read lower than the actual blood alcohol level 61% of the time, matched within a narrow margin 33% of the time, and read higher only 6% of the time. In other words, breathalyzers tend to underestimate your BAC slightly. This is why blood draws are often used as the definitive test in legal cases, while breath tests serve as a faster screening tool in the field.
Legal Limits in the United States
Every U.S. state sets 0.08% as the BAC threshold at which a driver is presumed to be under the influence. But legal consequences can begin below that number. In Illinois, for example, a BAC between 0.05% and 0.079% is classified as “driving impaired,” and a BAC below 0.05% still warrants caution. Many states have similar tiered systems, and penalties can apply even below 0.08% if an officer observes impaired driving behavior.
For drivers under 21, most states enforce zero-tolerance laws with limits as low as 0.00% to 0.02%. Commercial vehicle drivers are typically held to a 0.04% limit. Utah stands alone as the only state with a general driving limit of 0.05% rather than 0.08%.
Estimating Your Own BAC
Online BAC calculators and wallet charts can give you a rough idea based on your weight, sex, number of drinks, and time spent drinking. These tools use average absorption and elimination rates, so they’re approximations rather than precise measurements. They don’t account for food intake, medications, hydration, or individual metabolism.
A more practical way to think about it: for an average-sized man, two standard drinks in one hour will typically produce a BAC around 0.04% to 0.05%. For an average-sized woman, the same two drinks in an hour often push BAC closer to 0.06% to 0.07%. Three or four drinks in an hour can put most people at or above the legal limit. And because your liver clears only about one drink per hour, each additional drink stacks on top of what’s already in your system. If you stop drinking, expect your BAC to drop gradually over the following hours, not immediately.