A normal blood alcohol level is 0.00%, or very close to it. In a person who hasn’t been drinking, the amount of alcohol in the bloodstream is essentially zero. Your body does produce trace amounts of ethanol naturally through gut bacteria and yeast, but a healthy metabolism clears it before it ever reaches a measurable level in your blood. Any detectable BAC (blood alcohol concentration) indicates that alcohol has been consumed or, in rare cases, that a medical condition is interfering with normal processing.
How BAC Is Measured
BAC is expressed as a percentage representing grams of alcohol per 100 milliliters of blood. A BAC of 0.08% means there are 0.08 grams of alcohol in every 100 mL of your blood. This is the number police, hospitals, and labs all reference.
Breathalyzers estimate BAC indirectly by measuring alcohol in your breath. In a study of 61 volunteers with 242 paired tests, breathalyzer readings correlated with direct blood draws at a rate of 0.983 (nearly perfect), with 97% sensitivity and 93% specificity. Blood testing remains the gold standard, but a properly calibrated breathalyzer is close.
What Each BAC Level Feels Like
Even small amounts of alcohol push BAC above that 0.00% baseline, and the effects escalate quickly:
- 0.02%: Altered mood, slight relaxation, mild loss of judgment. One drink on an empty stomach can bring many people to this level.
- 0.05%: Lowered alertness, feeling uninhibited, noticeably impaired judgment. Reaction times begin to slow.
- 0.08%: Reduced muscle coordination, difficulty detecting danger, impaired reasoning. This is the legal driving limit in 49 U.S. states.
- 0.15% to 0.30%: Confusion, vomiting, drowsiness. Motor control deteriorates significantly.
- 0.30% to 0.40%: Alcohol poisoning territory. Loss of consciousness is likely.
- Above 0.40%: Risk of coma and death from respiratory arrest, meaning the body simply stops breathing.
The gap between “feeling fine” and “dangerously impaired” is narrower than most people realize. A BAC of 0.08% is only four hundredths of a percentage point above the level where judgment is already compromised.
Legal Limits in the United States
Every U.S. state sets the illegal per se BAC limit at 0.08%, with one exception: Utah lowered its limit to 0.05% in 2018. Colorado and New York also have separate “driving while ability impaired” laws that can apply at levels below 0.08%, and West Virginia can revoke a license above 0.05%.
For drivers under 21, all states enforce a limit of 0.02% or lower, effectively a zero-tolerance policy. People with prior DWI convictions face even tighter restrictions in some states, with limits set at 0.02% or 0.04%.
How Fast Your Body Processes Alcohol
Your liver breaks down alcohol at a fixed rate of about 0.015% BAC per hour. That rate doesn’t change with coffee, cold showers, or food after the fact. If your BAC is 0.08%, it takes roughly five and a half hours to return to 0.00%. At 0.15%, you’re looking at ten hours.
This is why people can still be over the legal limit the morning after heavy drinking. The math is straightforward but unforgiving: your liver simply can’t be rushed.
Why the Same Drink Hits People Differently
A single standard drink contains 0.6 ounces (14 grams) of pure alcohol, whether it comes as 12 ounces of beer at 5% ABV, 5 ounces of wine at 12%, or a 1.5-ounce shot of 80-proof liquor. But the same drink produces different BAC levels in different people, sometimes dramatically so.
Body composition is one of the biggest factors. Muscle tissue absorbs alcohol; fat does not. A person with more muscle and less body fat will distribute alcohol across more tissue, resulting in a lower BAC from the same drink. Someone who is less muscular reaches a higher BAC more quickly. Body weight matters for the same reason: a larger person has more blood volume to dilute the alcohol.
Biological sex plays a role too. Women tend to have higher body fat percentages, lower body water, and fewer of the stomach enzymes that begin breaking down alcohol before it enters the bloodstream. The result is that the same number of drinks typically produces a higher BAC in women than in men of similar weight.
Food makes a significant difference. Eating before or while drinking slows absorption because a valve at the bottom of the stomach closes during digestion, keeping alcohol from reaching the small intestine where it’s absorbed most rapidly. Greasy, high-protein, and fatty foods work best because they take longer to digest.
What Counts as Moderate Drinking
The CDC defines moderate alcohol use as two drinks or fewer per day for men and one drink or fewer per day for women. The Dietary Guidelines for Americans frames it as a ceiling, not a recommendation: adults of legal drinking age can choose not to drink or can drink within those limits. There is no suggested amount that’s considered beneficial, and the guidelines make clear that less is better from a health standpoint.
Staying within those limits generally keeps BAC in a range where impairment is mild and the liver can process the alcohol before the next day. But individual variation means even moderate drinking can push some people to a higher BAC than they expect, particularly if they’re smaller, haven’t eaten, or are drinking faster than one per hour.