A normal blood alcohol level is 0.00%. In a healthy person who has not consumed any alcohol, there should be no measurable alcohol in the bloodstream. Blood alcohol concentration, usually written as BAC, is expressed as a percentage representing grams of alcohol per 100 milliliters of blood. Any number above zero means alcohol is present in your system.
Understanding what different BAC levels mean, how quickly your body processes alcohol, and what pushes that number higher or lower can help you make better decisions about drinking.
What BAC Numbers Actually Mean
BAC is measured as a percentage, and even small increases from that 0.00% baseline produce noticeable changes in how you think, feel, and move. Here’s what happens as the number climbs:
- 0.02%: Altered mood, relaxation, and a slight loss of judgment. This is roughly where many people land after a single drink.
- 0.05%: You may feel uninhibited, with lowered alertness and noticeably impaired judgment.
- 0.08%: Reduced muscle coordination, difficulty detecting danger, and impaired reasoning. This is the legal driving limit for adults over 21 in all 50 U.S. states.
- 0.15% to 0.30%: Confusion, vomiting, and drowsiness. At this range, basic motor functions become unreliable.
- 0.30% to 0.40%: Alcohol poisoning territory. Loss of consciousness is likely, and the risk of death from respiratory failure climbs sharply.
The jump from 0.02% to 0.08% may look tiny on paper, but the difference in impairment is substantial. Reaction time, peripheral vision, and the ability to judge speed and distance all deteriorate well before a person “feels drunk.”
How Your Body Clears Alcohol
Your liver does the heavy lifting when it comes to eliminating alcohol. The average person clears alcohol at a rate of about 0.010% to 0.020% per hour. That means if your BAC reaches 0.10%, it could take anywhere from 5 to 10 hours to return to 0.00%. Nothing speeds this process up meaningfully. Coffee, cold showers, and food may change how you feel, but they do not lower your BAC any faster.
This clearance rate is relatively fixed because your liver can only process a limited amount of alcohol at a time. Drinking faster than your liver can keep up is exactly how BAC spikes into dangerous territory.
Why BAC Varies So Much Between People
Two people can drink the same amount and end up with very different BAC readings. Several factors explain why.
Body Size and Composition
Smaller people reach higher BAC levels faster because there is less tissue to absorb the alcohol. Body composition matters too. Alcohol distributes through lean tissue but not through fat. A person with a higher percentage of body fat will concentrate more alcohol in their lean tissue, pushing BAC higher even at the same body weight as someone with less fat.
Biological Sex
Women typically reach higher BAC levels than men after consuming the same amount of alcohol, even when body weight is similar. There are two reasons for this. Women produce about 40% less of the enzyme that breaks down alcohol in the stomach, so more alcohol enters the bloodstream intact. Women also tend to carry a higher percentage of body fat, which further concentrates alcohol in the remaining lean tissue.
Food in Your Stomach
Eating before or while drinking slows the absorption of alcohol into your bloodstream. Larger meals eaten close to the time of drinking can meaningfully lower your peak BAC. Food physically slows the stomach from emptying into the small intestine, where most alcohol absorption happens. Drinking on an empty stomach does the opposite, allowing alcohol to hit your bloodstream quickly and push BAC higher.
Legal BAC Limits for Driving
In the United States, 0.08% is the legal limit for drivers over 21. But several categories of drivers face stricter thresholds. Anyone under 21 faces a near-zero tolerance standard of 0.01% in many states. Commercial vehicle operators, including truck drivers and bus drivers, are held to 0.04%. Drivers on DUI probation are also typically limited to 0.01%.
Utah became the first state to lower its general limit to 0.05% in 2018, and other states have considered similar changes. It’s worth knowing that impairment begins well below 0.08%. At 0.05%, alertness and judgment are already compromised enough to affect driving ability.
Breath Tests vs. Blood Tests
BAC can be measured through a blood draw or estimated through a breath test (breathalyzer). Blood tests are more accurate. Breath tests carry a larger margin of error because the results can be skewed by factors that have nothing to do with drinking: certain diets, acid reflux, mouthwash, breath mints, and other products containing trace amounts of alcohol can all produce inflated readings.
If you’ve had a BAC test done as part of routine bloodwork or a medical screening and the result came back at 0.00%, that’s exactly where it should be. Any reading above zero in someone who hasn’t recently consumed alcohol could signal a medical issue worth investigating, such as a condition called auto-brewery syndrome where gut fermentation produces small amounts of alcohol internally. This is rare, but it does exist.
What a “Safe” BAC Looks Like
There is no universally “safe” BAC above zero. Even at 0.02%, judgment begins to shift. The only BAC that carries zero impairment risk is 0.00%. If you do drink, the factors that matter most are how quickly you consume alcohol relative to your body’s ability to clear it (roughly one standard drink per hour for most people), whether you’ve eaten, and your body size and composition. Keeping track of time between drinks is a far more reliable strategy than trying to guess how you feel, since subjective feelings of sobriety often lag behind actual impairment.