Blood alcohol level, or blood alcohol concentration (BAC), is a measurement of how much alcohol is in your bloodstream, expressed as a percentage. In the United States, a BAC of 0.08% is the legal limit for driving in every state except Utah, which lowered its limit to 0.05% in 2018. But impairment starts well before you hit any legal threshold, and dangerous levels begin lower than most people assume.
What BAC Numbers Actually Mean
BAC is measured as grams of alcohol per deciliter of blood. A BAC of 0.08% means there are 0.08 grams of pure alcohol in every 100 milliliters of your blood. That might sound tiny, but your brain is exquisitely sensitive to alcohol, so even small concentrations change how you think, react, and move.
A single “standard drink” in the U.S. contains 0.6 ounces (14 grams) of pure alcohol. That’s 12 ounces of regular beer at 5% alcohol, 5 ounces of wine at 12%, or 1.5 ounces of 80-proof liquor. One standard drink will raise most people’s BAC by roughly 0.02% to 0.03%, though this varies significantly based on body weight, sex, and whether you’ve eaten recently.
Effects at Each BAC Level
Alcohol’s effects follow a predictable pattern as BAC rises, though individual tolerance can mask how impaired you actually are.
- 0.02%: Altered mood, slight relaxation, and a mild loss of judgment. Most people feel loosened up but wouldn’t describe themselves as drunk.
- 0.05%: Lowered alertness, reduced inhibition, and noticeably impaired judgment. Reaction time starts slipping, which is why Utah and many countries set their legal driving limit here.
- 0.08%: Reduced muscle coordination, difficulty detecting danger, and impaired reasoning. This is the legal driving limit in 49 U.S. states.
- 0.10%: Slurred speech, slowed thinking, and clearly reduced reaction time.
- 0.15%: Nausea, vomiting, loss of balance, and significant loss of muscle control. This level is roughly double the legal limit and often triggers enhanced penalties in DUI cases.
- 0.15% to 0.30%: Confusion, drowsiness, and continued vomiting. Blackouts are common in this range.
- 0.30% to 0.40%: Alcohol poisoning territory. Loss of consciousness is likely, and your body’s basic functions start failing.
- Above 0.40%: Risk of coma and death from respiratory arrest, where breathing simply stops.
Legal Limits for Driving
Every U.S. state sets 0.08% as the per se limit for standard drivers, meaning you can be charged with impaired driving at or above that number regardless of how you appear to be functioning. Utah is the sole exception, having dropped to 0.05% at the end of 2018. A NHTSA-funded study found that Utah’s fatal crash rate fell by 19.8% in the first year under the lower limit, compared to a 5.6% reduction across the rest of the country during the same period.
Commercial drivers (truckers, bus operators) face a federal limit of 0.04% in all states. Drivers under 21 fall under “zero tolerance” laws, which typically set the limit at 0.02% or lower, depending on the state.
Internationally, limits vary widely. Most of Europe, Australia, and much of Asia use 0.05%. Brazil dropped its limit to 0.02% for all drivers in 2008 and saw a 45% decrease in drivers testing positive at roadside checks. British Columbia, Canada, introduced administrative penalties for drivers between 0.05% and 0.08% in 2010, and alcohol-related fatal crashes fell 40% in the following year.
How Fast Your Body Clears Alcohol
Your liver does nearly all the work of breaking down alcohol, and it operates on a fixed schedule that you cannot speed up. The average person eliminates alcohol at a rate of 0.010% to 0.020% BAC per hour. That means someone at 0.10% would need anywhere from 5 to 10 hours to reach 0.00%.
This is why a night of heavy drinking can leave you over the legal limit the next morning. If you stop drinking at midnight with a BAC of 0.15%, you may not reach 0.08% until 4 to 7 a.m., and you won’t be fully sober until well into the afternoon. Coffee, cold showers, and food do not change this timeline. Caffeine may make you feel more alert, but it has no effect on how quickly your body metabolizes alcohol. Your BAC stays exactly the same whether you drink a pot of coffee or go to sleep.
What Changes Your BAC
Two people can drink the same amount and end up at very different BAC levels. Body weight is the biggest factor: a larger body has more blood volume, so the same amount of alcohol gets diluted more. Sex matters too, because women generally have a higher percentage of body fat and less body water than men of the same weight, which means alcohol concentrates more in the bloodstream.
Food in your stomach slows the rate at which alcohol passes into your small intestine, where most absorption happens. Drinking on an empty stomach can produce a BAC peak roughly twice as high as drinking the same amount with a full meal. The type of drink matters less than people think. A standard beer, glass of wine, and shot of liquor all contain the same amount of pure alcohol. Carbonated mixers can speed absorption slightly, but the difference is modest.
Medications can also shift the equation. Anything that affects liver function or competes for the same metabolic pathways can slow alcohol processing, effectively raising your BAC for longer than expected.
How BAC Is Measured
There are two main methods: breath testing and blood draws. Roadside breathalyzers estimate BAC by measuring alcohol vapor in your exhaled breath and converting it to an equivalent blood concentration. They’re convenient but imperfect. Research comparing breath tests to actual venous blood draws found that breathalyzers read about 15% lower than the true blood alcohol level on average, with significant random variation from one test to the next. This is why breath tests are typically used for screening at the roadside, while a blood draw at a hospital or police station provides the more accurate number used as evidence.
Urine tests can also detect alcohol but are less precise for determining a current BAC, since urine reflects what your body processed over the previous few hours rather than what’s in your blood right now.
When BAC Becomes a Medical Emergency
Alcohol poisoning generally begins in the 0.30% to 0.40% range, though it can occur at lower levels in people with low tolerance, small body size, or those who have taken other sedating substances. At these concentrations, the parts of your brain that control breathing and heart rate start to shut down. Loss of consciousness is common, and vomiting while unconscious creates a serious choking risk.
Above 0.40%, the risk of coma and death from respiratory arrest rises sharply. Because alcohol continues to absorb from the stomach and intestine even after someone stops drinking (or passes out), BAC can keep climbing for 30 to 90 minutes after the last sip. Someone who seems “just drunk” can deteriorate quickly. Irregular breathing, cold or bluish skin, and unresponsiveness are signs the situation has moved beyond intoxication into a life-threatening emergency.