Blood alcohol, formally called blood alcohol concentration (BAC), is the percentage of alcohol in your bloodstream at any given moment. It’s expressed as a decimal: a BAC of 0.08% means there are 0.08 grams of alcohol per deciliter of blood. That number determines how impaired you are, whether you’re legally allowed to drive, and at high enough levels, whether your body can keep functioning safely.
How Alcohol Enters Your Blood
When you drink, alcohol doesn’t need to be digested like food. It passes through the walls of your stomach and intestines directly into your bloodstream. About 20% is absorbed through the stomach lining, while the remaining 80% is absorbed in the small intestine, where it enters the blood much faster.
This is why eating matters so much. When food is in your stomach, a valve at the bottom stays partially closed, releasing alcohol into the small intestine gradually. That slows absorption and produces a lower, more gradual peak BAC. On an empty stomach, alcohol passes straight into the small intestine and hits your bloodstream quickly, which is why drinking without eating can make you feel intoxicated surprisingly fast.
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 size and weight. A smaller person has less blood volume to dilute the alcohol, so BAC rises faster.
- Body composition. Muscle tissue contains more water than fat tissue. Since alcohol dissolves in water, people with more muscle and less body fat distribute alcohol more evenly, resulting in a lower BAC per drink.
- Biological sex. Women generally absorb more alcohol and take longer to process it than men, even at the same body weight. This is partly because women tend to have a higher ratio of body fat to water. After drinking the same amount, women typically reach a higher BAC than men.
- Food and hydration. A recent meal and adequate water intake both slow absorption and help keep BAC from spiking.
What Each BAC Level Feels Like
BAC effects follow a fairly predictable pattern, though individual tolerance can shift when symptoms become noticeable. The National Highway Traffic Safety Administration breaks it down this way:
At 0.02%, most people feel slightly relaxed and warm. Mood shifts a little, and there’s a subtle decline in the ability to track moving objects or divide attention between two tasks. You might not feel “drunk” at all.
At 0.05%, inhibitions drop noticeably. Judgment is impaired, alertness decreases, and small-muscle control starts to slip, making it harder to focus your eyes precisely. Coordination is reduced, and steering a vehicle becomes measurably more difficult. This is the legal driving limit in Utah.
At 0.08%, the legal limit in the other 49 states, muscle coordination is clearly affected. Balance, speech, vision, reaction time, and hearing all deteriorate. Short-term memory loss begins, self-control weakens, and the ability to process visual information or detect danger drops significantly. Most people feel obviously impaired at this level.
At 0.15%, you have far less muscle control than normal. Balance is seriously compromised, and vomiting is common unless the person reached this level very slowly or has built up a high tolerance. Mood can swing unpredictably.
At 0.30% to 0.40%, alcohol poisoning is likely. Loss of consciousness occurs, and the body’s basic protective reflexes start to fail. Above 0.40%, coma and death from respiratory arrest (the body simply stops breathing) become real possibilities.
The Legal Limit and Why It Exists
Every U.S. state sets a legal per se BAC limit for drivers. In 49 states, that limit is 0.08%. Utah lowered its limit to 0.05% in 2018. For drivers under 21, all states enforce a limit of 0.02% or lower, effectively a zero-tolerance policy. Commercial drivers face stricter limits as well, typically 0.04%.
“Per se” means that reaching or exceeding that BAC is illegal on its own, regardless of whether you appear impaired. You can still be charged with impaired driving below the legal limit if an officer observes signs of impairment.
How BAC Is Tested
The two most common methods are breath tests and blood tests, and they differ in both convenience and accuracy.
A breath test uses a machine (commonly called a breathalyzer) that analyzes alcohol molecules in exhaled air and estimates your BAC from that sample. It’s fast, noninvasive, and produces results on the spot. The tradeoff is a wider margin of error. Factors like diet, certain health conditions, mouthwash, mints, or other alcohol-containing products can skew the reading.
A blood test involves drawing a sample that’s sent to a lab for analysis. Results take longer, but they’re generally more accurate because the measurement isn’t influenced by what’s in your mouth or lungs. Blood draws are typically performed by a technician at a police station or by hospital staff if medical treatment is involved.
Urine tests can also detect alcohol but are less commonly used for determining precise BAC levels. They’re more useful for confirming whether someone has consumed alcohol recently rather than measuring current impairment.
How Your Body Clears Alcohol
Your liver does nearly all the work of breaking down alcohol, and it operates at a fixed pace: roughly one standard drink per hour. A standard drink is 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of distilled spirits. No matter how much you’ve had, the liver can’t speed up to compensate.
This is why time is the only reliable way to sober up. Coffee, cold showers, food, and exercise don’t accelerate the process. They might make you feel more alert, but your BAC drops at the same steady rate regardless. If you’ve had four drinks in two hours, it will take your liver approximately four hours from your last drink to fully clear the alcohol, though your BAC will be declining throughout that window.
When BAC Becomes a Medical Emergency
Alcohol poisoning typically occurs in the 0.30% to 0.40% range, though it can happen at lower levels depending on the person’s size, tolerance, and health. At these concentrations, the brain’s ability to control basic life-support functions, including breathing and heart rate, is suppressed.
Warning signs include vomiting while unconscious or semiconscious, slow or irregular breathing (fewer than eight breaths per minute), pale or bluish skin, low body temperature, and an inability to be woken up. Because the stomach continues absorbing alcohol even after someone stops drinking, BAC can keep rising after a person passes out. Leaving someone to “sleep it off” when they’re showing these signs is dangerous.