Blood alcohol content (BAC) is the amount of alcohol in your bloodstream, expressed as a percentage. A BAC of 0.08%, the legal driving limit in 49 U.S. states, means there are 0.08 grams of alcohol per 100 milliliters of blood. That number rises and falls based on how quickly you drink, your body size, your biological sex, and whether you’ve eaten.
How BAC Is Measured
BAC can be measured directly through a blood draw or estimated indirectly through breath, urine, or saliva tests. Breathalyzers are the most common field tool because they give near-instant results, but a lab blood test is the most accurate. Results are reported either as a percentage (0.08% BAC) or as grams per milliliter (0.08 g/100 mL), which represent the same value.
There’s also a formula for estimating BAC without any test at all. Developed by Swedish researcher Erik Widmark in the 1930s, it divides the total grams of alcohol consumed by your body weight multiplied by a distribution factor: 0.7 for men and 0.6 for women at a normal weight. Slim individuals use a slightly higher factor (0.8 for men, 0.7 for women), and those carrying more body fat use a lower one (0.6 for men, 0.5 for women). Online BAC calculators are based on variations of this formula, though they’re only rough estimates.
What Each BAC Level Feels Like
The National Highway Traffic Safety Administration breaks impairment into ranges that show just how quickly alcohol degrades your abilities, even at levels well below the legal limit.
At 0.02%, roughly one drink, most people feel slightly warmer and more relaxed. Mood shifts slightly, and the ability to track a moving object or divide attention between two tasks begins to decline. You’d likely feel fine. You wouldn’t be.
At 0.05%, behavior becomes more exaggerated and inhibitions drop. Small-muscle control weakens, making it harder to focus your eyes. Judgment is impaired, alertness drops, and coordination suffers. In a driving context, steering becomes more difficult and response time to emergencies slows noticeably.
At 0.08%, the legal limit in most of the U.S., muscle coordination is clearly affected. Balance, speech, vision, reaction time, and hearing all deteriorate. Self-control, reasoning, and short-term memory are impaired. Processing information, like reading a road sign while checking your mirrors, becomes significantly harder.
At 0.15%, nearly twice the legal limit, muscle control is severely reduced. Balance becomes unreliable. Vomiting is common unless the person has built a high tolerance or reached this level very gradually.
Between 0.16% and 0.30%, walking and speaking become difficult. Drowsiness, confusion, nausea, memory blackouts, vomiting, and loss of consciousness can all occur. Above 0.31%, the situation turns life-threatening. Breathing may become irregular, coma is possible, and death can result.
Why the Same Drinks Hit People Differently
Two people can drink identical amounts and end up at very different BAC levels. The biggest factors are body weight, biological sex, body composition, and food intake.
Body weight matters because alcohol distributes through the water in your body. A larger person has more water to dilute the alcohol, producing a lower BAC from the same number of drinks. Body fat plays a role too, because fat tissue contains very little water. Someone with a higher body fat percentage has less water available to dilute alcohol, so their BAC climbs higher.
Biological sex creates a consistent gap. A 150-pound man who drinks four beers will reach a BAC of roughly 0.10%. A 150-pound woman drinking the same amount will typically reach about 0.12%. This difference comes from two things: women generally carry a higher proportion of body fat (and therefore less body water), and women produce lower levels of the stomach enzyme that begins breaking down alcohol before it reaches the bloodstream.
Food in your stomach is one of the most controllable factors. Eating before or while drinking slows gastric emptying, which delays alcohol’s arrival in the small intestine, where most absorption happens. That delay gives your stomach more time to begin metabolizing alcohol before it reaches your blood, resulting in a lower and later peak BAC. Drinking on an empty stomach does the opposite: alcohol passes quickly into the small intestine and your BAC spikes faster and higher.
How Your Body Clears Alcohol
Your liver does the vast majority of the work. It produces an enzyme that breaks down alcohol at a remarkably steady rate: about 0.015% BAC per hour. That rate barely changes regardless of your size, fitness level, or how much coffee you drink.
In practical terms, this means if you stop drinking at a BAC of 0.08%, it will take roughly five and a half hours to reach 0.00%. At 0.15%, you’re looking at about ten hours. Cold showers, food, and exercise do not speed this up. Only time does. This is why people can still be over the legal limit the morning after a night of heavy drinking.
Because your body clears alcohol at a fixed pace but absorbs it at a variable one, drinking speed matters enormously. Spacing drinks out gives your liver a chance to keep up. Drinking rapidly overwhelms it, and BAC climbs steeply.
Legal Limits in the U.S. and Worldwide
Every U.S. state sets a legal BAC limit for driving. Forty-nine states draw the line at 0.08%. Utah is the sole exception, having lowered its limit to 0.05% in December 2018. For commercial drivers, the federal limit is 0.04%, and for drivers under 21, most states enforce zero-tolerance laws that make any detectable BAC illegal.
Globally, limits vary widely. Most of Western Europe, including Germany, France, Spain, Italy, and the Netherlands, sets the limit at 0.05%. The UK (excluding Scotland) is an outlier in Europe at 0.08%, while Scotland matches the continental standard at 0.05%. Several European countries enforce zero tolerance, including Czechia, Hungary, Romania, and Slovakia.
In Asia, the range is even broader. China and South Korea set limits at 0.02% and 0.03% respectively. Japan also uses 0.03%. Many countries in the Middle East and parts of South Asia either prohibit alcohol entirely or have no defined BAC limit for drivers. Australia uses 0.05% as its general limit, with zero tolerance for certain categories of drivers.
The trend in recent decades has been toward lower limits. Research consistently shows that impairment begins well below 0.08%, which is why public health organizations in the U.S. have recommended following Utah’s lead and adopting 0.05% nationwide.
Estimating BAC From Standard Drinks
A “standard drink” in the U.S. contains about 14 grams of pure alcohol. That’s one 12-ounce beer at 5% alcohol, one 5-ounce glass of wine at 12%, or one 1.5-ounce shot of liquor at 40%. Each standard drink raises BAC by roughly 0.02% to 0.03% in an average-sized person, though the exact amount depends on the factors described above.
Keep in mind that many real-world drinks exceed these standard sizes. A pint of craft beer at 7% alcohol is closer to one and a half standard drinks. A generous pour of wine can easily be eight ounces instead of five. Cocktails often contain two or more shots. When people underestimate their BAC, it’s usually because they’re counting drinks rather than counting the actual alcohol in them.