BAC stands for blood alcohol concentration, and it measures how much alcohol is currently in your bloodstream. It’s expressed as a percentage: a BAC of 0.08% means that 0.08 grams of alcohol are present in every 100 milliliters of your blood. This single number is what law enforcement uses to determine legal intoxication, what hospitals use to assess alcohol poisoning, and what your body experiences as everything from a slight buzz to a life-threatening emergency.
How BAC Is Measured
There are two main ways to measure BAC: a blood draw and a breathalyzer. A blood test gives a direct reading, reported either as a percentage (0.08%) or in grams per milliliter (0.08 g/100 mL). Both notations mean the same thing.
A breathalyzer works indirectly. Alcohol in your blood crosses into the tiny air sacs in your lungs, so when you exhale, your breath carries a small but measurable amount of alcohol vapor. The device converts that breath reading into an estimated blood level using a standard ratio: on average, 1 milliliter of blood contains 2,100 times more alcohol than 1 milliliter of air from the lungs. This 2,100:1 conversion is built into every breathalyzer. In reality, the ratio can range from 1,500:1 to 3,000:1 depending on the individual, which is one reason breathalyzer results are sometimes challenged in court.
What Different BAC Levels Feel Like
BAC effects follow a fairly predictable curve, though individual tolerance can shift the experience somewhat. Here’s a general breakdown:
- 0.02% to 0.03%: A mild sense of relaxation. Judgment is very slightly loosened, but most people feel essentially normal.
- 0.05% to 0.06%: Lowered inhibitions, warmer mood, and slightly impaired coordination. Reaction time begins to slow. Many countries set their legal driving limit here.
- 0.08%: The legal limit for driving in the United States and the UK. At this level, muscle coordination is noticeably reduced, balance is off, and your ability to detect danger or react to it drops significantly.
- 0.10% to 0.12%: Clear deterioration in reaction time and motor control. Speech becomes slurred. Thinking and judgment are significantly impaired.
- 0.15%: A high level of impairment. Walking becomes difficult, and vomiting is common as the body tries to rid itself of alcohol. Emotional control is poor.
- 0.20% to 0.25%: Confusion, disorientation, and the possibility of blacking out. At these levels, the gag reflex may be suppressed, which creates a real choking risk if vomiting occurs.
- 0.30% and above: Loss of consciousness is likely. Breathing can slow dangerously, body temperature drops, and the risk of fatal alcohol poisoning becomes serious.
- 0.40% and above: Potentially fatal for most people. Respiratory failure and cardiac arrest are possible.
Why the Same Drinks Hit People Differently
Two people can drink the same amount of alcohol and end up with very different BAC readings. Several biological factors explain why.
Body weight is the most obvious one. Alcohol distributes through the water in your body, so a larger person with more total body water will dilute the same amount of alcohol into a lower concentration. But weight alone doesn’t tell the whole story. Body composition matters just as much. Fat tissue doesn’t absorb alcohol the way muscle and water-rich tissue do, so a person with a higher body fat percentage will have a higher BAC than a more muscular person of the same weight after identical drinks.
Biological sex plays a role beyond body size. Women generally have higher body fat percentages, lower total body water, and higher estrogen levels, all of which keep more alcohol circulating in the bloodstream rather than being absorbed into tissues. Men also tend to produce more of the stomach enzyme that begins breaking down alcohol before it even reaches the bloodstream. The net result is that women typically reach a higher BAC faster from the same number of drinks.
Food makes a substantial difference. When you eat before or while drinking, the valve at the bottom of your stomach closes to allow digestion. This traps alcohol in the stomach longer and slows its passage into the small intestine, where most absorption happens. Greasy, fatty, and high-protein foods are especially effective at slowing absorption because they take longer to digest. Drinking on an empty stomach sends alcohol straight to the small intestine, causing a faster and higher BAC spike.
How Your Body Eliminates Alcohol
Your liver does the vast majority of the work. Once your BAC peaks, your body eliminates alcohol at a roughly constant rate, typically between 0.010% and 0.020% per hour. A common midpoint estimate is about 0.015% per hour, though this varies by individual.
To put that in practical terms: if your BAC reaches 0.10%, it will take somewhere between 5 and 10 hours to return to 0.00%. Coffee, cold showers, and exercise don’t speed this up. The liver processes alcohol on its own timeline, and nothing you do can meaningfully accelerate it. This is why someone who stops drinking at midnight can still be legally impaired the next morning.
Estimating BAC With the Widmark Formula
There’s a well-known formula for estimating BAC without a test. Developed by Swedish chemist Erik Widmark, it divides the total grams of alcohol consumed by your body weight and a distribution factor that accounts for how alcohol spreads through your body. The distribution factor averages about 0.68 for men and 0.55 for women. For someone who is heavier or more muscular, the factor shifts upward (as high as 0.8 for slim men or 0.7 for slim women). For someone with more body fat, it drops (to about 0.6 for obese men, 0.5 for obese women).
Online BAC calculators are built on variations of this formula. They can give you a rough estimate, but they can’t account for your individual metabolism, how fast you drank, or how much food was in your stomach. Treat any calculator result as an approximation, not a guarantee of whether you’re safe to drive.
Legal Limits Around the World
In the United States, 0.08% is the legal BAC limit for drivers 21 and older in all 50 states. Commercial drivers face a stricter 0.04% limit, and drivers under 21 are held to zero-tolerance laws that effectively set the limit at 0.00% to 0.02%, depending on the state.
Most of Europe sets the standard limit at 0.05% (0.5 grams per liter), including France, Germany, Austria, Italy, Spain, and Ireland. Several countries go further. Sweden, Norway, and Poland use a 0.02% limit. The Czech Republic, Hungary, Romania, and Slovakia enforce a strict 0.00% for all drivers. The UK (excluding Scotland) is an outlier in Europe, matching the U.S. at 0.08%. Scotland lowered its limit to 0.05% in 2014.
Many European countries also impose lower limits for commercial and novice drivers. In Germany and Italy, for example, new drivers and professional drivers face a 0.00% limit even though the general population is allowed 0.05%. The global trend has been toward stricter limits, with growing evidence that impairment begins well below 0.08%.
Why BAC Matters More Than Drink Counts
Counting drinks is a poor substitute for understanding BAC. A “standard drink” in the U.S. contains about 14 grams of pure alcohol, roughly the amount in 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of distilled spirits. But pour sizes at bars and homes vary widely, craft beers can contain double the alcohol of a light beer, and cocktails often contain multiple shots. Two people having “three drinks” could easily have consumed very different amounts of actual alcohol.
BAC captures what’s actually happening in your body. It integrates every variable: how much you drank, what you drank, how fast you drank it, your size, your sex, your metabolism, and whether you ate. It’s the single number that correlates most directly with impairment, legal risk, and medical danger.