BAC stands for blood alcohol concentration (sometimes called blood alcohol content). It’s the percentage of alcohol in your bloodstream, expressed as a simple number like 0.08%. That specific number, 0.08%, is the legal driving limit in every U.S. state except Utah, which lowered its limit to 0.05% in 2018. But BAC matters beyond the legal context: it’s the most reliable way to gauge how impaired someone actually is after drinking.
How BAC Is Measured
BAC is expressed as a percentage of alcohol by weight in your blood. A BAC of 0.08% means there are 0.08 grams of alcohol per 100 milliliters of blood. That sounds tiny, but even small concentrations produce noticeable effects on coordination, judgment, and reaction time.
There are three common ways to measure it. A blood draw is the most accurate method and the gold standard in medical and legal settings. Breathalyzers estimate BAC by measuring alcohol in exhaled air. They’re convenient but tend to read slightly low. A study comparing over 400 paired breath and blood samples found that breathalyzers underestimated the actual blood result by more than 0.01 about 61% of the time, matched it within 0.01 about 33% of the time, and overestimated it only 6% of the time. Urine tests can also detect alcohol but are less precise for determining a current BAC level.
What Each BAC Level Feels Like
The effects of alcohol follow a fairly predictable progression as BAC rises:
- 0.02%: Altered mood, mild relaxation, and a slight loosening of judgment. Most people feel “a little buzzed” but appear normal to others.
- 0.05%: Lowered alertness, reduced inhibitions, and more noticeably impaired judgment. This is where Utah’s legal driving limit kicks in.
- 0.08%: Reduced muscle coordination, difficulty detecting danger, and clearly impaired reasoning. This is the legal limit in 49 states.
- 0.15% to 0.30%: Confusion, vomiting, and drowsiness. Walking and speaking become difficult. Blackouts are common in this range.
- 0.30% to 0.40%: Alcohol poisoning territory. Loss of consciousness, dangerously slowed breathing, and risk of coma. Levels above 0.31% can be fatal.
These ranges are averages. Someone who rarely drinks may feel significant effects at 0.05%, while a person with high tolerance might seem functional at 0.15%, even though their reflexes and decision-making are severely compromised regardless of how they feel.
What Determines Your BAC
Two people can drink the same amount and end up with very different BAC levels. Several biological factors explain why.
Body weight is the most obvious one. A larger body has more blood volume and water to dilute the alcohol, so a 200-pound person will generally reach a lower BAC than a 130-pound person after the same number of drinks. Sex matters too: women tend to reach higher peak BAC levels than men after consuming the same amount of alcohol, partly because women typically carry a higher percentage of body fat (which doesn’t absorb alcohol) and have less of the enzyme that breaks alcohol down in the stomach.
Food plays a surprisingly large role. Eating before or while drinking slows alcohol absorption and lowers your peak BAC. A high-energy meal blunts the spike more than a light snack. Interestingly, meal composition matters: high-protein meals produce the lowest peak BAC, while high-fat meals produce the highest among equal-calorie options, likely because fat slows stomach emptying in a way that still allows steady alcohol absorption over time.
Other factors include how quickly you drink, whether you’re dehydrated, and individual genetic variation in how efficiently your liver processes alcohol.
How Your Body Processes Alcohol
Your liver does the heavy lifting. The average 70-kilogram (about 154-pound) person can metabolize roughly 7 grams of alcohol per hour, which works out to approximately one standard drink per hour. A standard drink in the United States contains 14 grams of pure alcohol, which is the amount in a 12-ounce beer, a 5-ounce glass of wine, or a 1.5-ounce shot of liquor.
This means if you have two drinks in an hour, your body can only clear about half of that alcohol in the same timeframe. The rest stays in your bloodstream, and your BAC keeps climbing. A common rule of thumb is that BAC drops by roughly 0.015% per hour, though individual metabolism varies. Nothing speeds this process up: not coffee, not cold showers, not food after the fact. Only time works.
How BAC Is Estimated Before Testing
Forensic scientists and legal experts sometimes estimate BAC using a formula developed by Swedish researcher Erik Widmark in the 1930s. The basic idea is straightforward: take the mass of alcohol consumed (in grams), divide it by the person’s body mass and a distribution factor that accounts for body composition, then subtract the amount of alcohol eliminated over time.
The distribution factor is higher for men (typically around 0.68) than for women (around 0.55), reflecting differences in body water content. The elimination rate varies from person to person but averages around 15 milligrams per deciliter per hour.
These calculations are useful for courtroom reconstructions or rough personal estimates, but they carry real uncertainty. Individual variation in absorption, metabolism, and body composition means the formula can produce results that differ from actual measured BAC by a meaningful margin.
BAC and the Law
Every U.S. state treats driving at or above the legal BAC limit as a criminal offense, commonly called DUI or DWI. The 0.08% threshold has been the nationwide standard since 2004, when the last holdout states adopted it under pressure from federal highway funding rules. Utah broke from this consensus in 2018 by lowering its limit to 0.05%, a level at which most people already experience measurable impairment in reaction time and visual tracking.
For commercial drivers, the federal limit is 0.04%. For drivers under 21, most states enforce a zero-tolerance policy, meaning any detectable BAC is illegal. It’s also worth knowing that you can be charged with impaired driving below 0.08% if an officer determines your ability to drive is compromised. The legal limit is a threshold for automatic charges, not a safe level.