Blood alcohol concentration, or BAC, is a measurement of how much alcohol is currently in your bloodstream, expressed as a percentage of your blood volume. A BAC of 0.08% means there are 0.08 grams of alcohol for every 100 milliliters of blood. That number rises as you drink, peaks once your body finishes absorbing the alcohol, and then gradually falls as your liver breaks it down. Understanding how BAC works means understanding each of those stages and the many factors that shift the numbers up or down.
How Alcohol Gets Into Your Blood
When you take a drink, the alcohol doesn’t need to be digested the way food does. It passes directly through the walls of your stomach and small intestine into your bloodstream. The key detail: absorption from the stomach is slow, while absorption from the small intestine is fast. This means anything that controls how quickly alcohol leaves your stomach has a major effect on how fast your BAC rises.
Eating before or while you drink slows gastric emptying, the process of food moving from your stomach into your small intestine. A full stomach keeps alcohol trapped longer, releasing it gradually. That’s why drinking on an empty stomach produces a noticeably higher and faster BAC peak compared to drinking the same amount with a meal. The total alcohol absorbed is roughly the same either way, but the rate changes dramatically, and rate is what determines how impaired you feel at any given moment.
How Your Liver Breaks It Down
Once alcohol reaches your bloodstream, your liver does most of the work clearing it out. The process happens in two main steps. First, an enzyme called alcohol dehydrogenase converts ethanol into acetaldehyde, a toxic compound and known carcinogen. Then a second enzyme, aldehyde dehydrogenase, quickly converts that acetaldehyde into acetate, a much less harmful substance your body can break down into water and carbon dioxide.
Your liver can only process alcohol at a relatively fixed pace. The average person with a body weight around 154 pounds (70 kg) clears roughly 7 grams of alcohol per hour, which is about one standard drink per hour. But there’s significant individual variation. Some people metabolize alcohol three to four times faster or slower than others due to genetics, liver health, and other biological factors. A backup pathway involving a different enzyme kicks in after heavy drinking, but for moderate consumption, that primary two-step process handles nearly all of it.
What Counts as One Drink
In the United States, one standard drink contains about 14 grams of pure alcohol, which equals roughly 0.6 fluid ounces. That’s the amount in a 12-ounce beer at 5% alcohol, a 5-ounce glass of wine at 12%, or a 1.5-ounce shot of liquor at 40%. These are smaller servings than many people actually pour, which is one reason BAC often climbs higher than expected. A large glass of wine at a restaurant can easily be 8 or 9 ounces, meaning it’s closer to two standard drinks.
Why BAC Varies Between People
Two people can drink the same amount of alcohol and end up with very different BAC readings. The most important factor is body water volume. Alcohol dissolves in water, so the more water your body contains, the more diluted the alcohol becomes. Larger people generally have more total body water, which means a lower BAC for the same number of drinks.
Sex plays a role here because men and women tend to differ in body composition. The Widmark formula, the standard equation used to estimate BAC, accounts for this with a distribution factor that varies by sex and body mass index. For men, this factor is calculated as roughly 1.0181 minus 0.01213 times BMI. For women, it’s 0.9367 minus 0.01240 times BMI. In practical terms, a woman will typically reach a higher BAC than a man of the same weight after consuming the same amount of alcohol.
Age matters too. Research comparing younger and older women who drank identical doses found that older women reached significantly higher peak BAC levels (averaging 0.097% compared to about 0.082% in younger women). The reasons are complex and not fully explained by body water changes alone, but the pattern is consistent: as you age, the same amount of alcohol produces a higher reading.
What the Numbers Feel Like
BAC is not just a legal abstraction. Each threshold corresponds to real, measurable changes in how your brain and body perform.
- 0.02%: Subtle mood changes, slight warmth, minor decline in your ability to track moving objects or split your attention between two tasks.
- 0.05%: Lowered alertness, reduced coordination, exaggerated behavior, difficulty focusing your eyes. Reaction time in driving situations starts to suffer.
- 0.08%: Poor muscle coordination affecting balance, speech, and vision. Impaired judgment, self-control, reasoning, and short-term memory. Reduced ability to process information or detect danger.
- 0.10%: Clear deterioration in reaction time. Slurred speech, poor coordination, slowed thinking. Difficulty maintaining lane position while driving.
- 0.15%: Major loss of muscle control and balance. Vomiting is common unless tolerance has built up. Substantial impairment in attention, vehicle control, and the ability to process what you see and hear.
These effects are cumulative and progressive. The jump from 0.05% to 0.08% involves a meaningful decline in cognitive function, not just a slight worsening.
How Breathalyzers Estimate BAC
When you exhale, a small amount of alcohol evaporates from your blood into the air in your lungs. Breathalyzers exploit this by measuring the alcohol concentration in your breath and converting it to a blood alcohol estimate using a fixed ratio of 2,100 to 1. That ratio means 2,100 milliliters of breath air contains roughly the same amount of alcohol as 1 milliliter of blood.
Most modern breathalyzers use a fuel cell sensor. Breath passes over a platinum electrode, where alcohol undergoes a chemical reaction that generates a tiny electrical current. The stronger the current, the more alcohol is present. This technology is reliable under normal conditions, but the 2,100:1 ratio is an average. Individual variation in lung physiology, body temperature, and breathing patterns can nudge results slightly higher or lower than your true blood level.
The Descending Limb Problem
One of the more dangerous aspects of BAC is how your perception of impairment shifts over time. As your BAC climbs (the ascending limb), you feel increasingly intoxicated. But once it peaks and starts falling (the descending limb), something misleading happens: you start to feel more sober even though your BAC may still be high. This is called acute tolerance.
Studies have shown that people report feeling significantly less drunk when their BAC is falling compared to when it was rising, even at identical BAC levels. The problem is that actual performance, including driving ability, short-term memory, and reaction accuracy, remains impaired on the descending limb. This gap between how sober you feel and how impaired you actually are leads to overconfidence. Research consistently finds that people are more willing to drive as their BAC falls, precisely when they’re most likely to misjudge their own impairment.
Legal BAC Limits
In the United States, 0.08% is the per se legal limit for drivers 21 and older in all 50 states, meaning you can be charged with impaired driving at or above that level regardless of how you appear to be functioning. Many states also have lower thresholds for commercial drivers (typically 0.04%) and zero-tolerance laws for drivers under 21.
Internationally, limits vary widely. Most of Europe sets the standard limit at 0.05%, including France, Germany, Austria, and Spain. Several countries are stricter: Sweden, Norway, and Poland set their limit at 0.02%, while the Czech Republic, Hungary, Romania, and Slovakia enforce a true zero-tolerance policy with a 0.0% limit. The United Kingdom is an outlier in Europe at 0.08%, though Scotland independently lowered its limit to 0.05%. Many European countries also impose tighter limits on commercial and novice drivers, sometimes at 0.0%.
These legal thresholds don’t represent safe levels of impairment. Measurable cognitive and motor deficits begin at 0.02%, well below any country’s legal limit. The legal line is a policy choice, not a biological one.
How Long It Takes to Return to Zero
Since your liver processes roughly one standard drink per hour, you can estimate your timeline accordingly. If you stop drinking at a BAC of 0.08%, it will take approximately five to six hours to reach 0.00% for an average metabolizer. But given the three- to four-fold variation in elimination rates across the population, a slow metabolizer in the same situation could need considerably longer.
Nothing speeds up this process. Coffee, cold showers, food, and exercise do not increase your liver’s metabolic rate. They may make you feel more alert, but your BAC drops at the same pace regardless. The only variable that changes elimination speed is your individual biology, and you can’t adjust that on a given night.