A normal blood alcohol concentration (BAC) is 0.00%, meaning no measurable alcohol in your bloodstream. Any BAC above zero reflects alcohol you’ve consumed that your body hasn’t yet processed. BAC is expressed as a percentage of alcohol by volume in your blood, so 0.08% means 0.08 grams of alcohol per 100 milliliters of blood.
What BAC Numbers Actually Mean
BAC is a sliding scale from sober to life-threatening. Here’s what happens at each range:
- 0.00% to 0.01%: Sober. No measurable impairment.
- 0.02% to 0.03%: Mild relaxation and slight lightheadedness. Most people feel a subtle mood lift but show little outward change.
- 0.04% to 0.06%: Lowered inhibitions, warmth, and mild impairment in judgment and fine motor control. Reaction time starts to slow.
- 0.07% to 0.09%: Noticeable impairment in balance, speech, and reaction time. Judgment and self-control are clearly reduced.
- 0.10% to 0.15%: Significant impairment. Slurred speech, poor coordination, and blurred vision are common. Nausea may begin.
- 0.16% to 0.29%: Severe impairment. Disorientation, vomiting, and blackouts become likely. Walking without help is difficult.
- 0.30% to 0.40%: Alcohol poisoning territory. Loss of consciousness, dangerously slowed breathing, and a real risk of death.
- Above 0.40%: Potentially fatal. Coma and death from respiratory arrest are possible at this level.
The Legal Limit Across the U.S.
As of January 2025, every U.S. state sets the legal driving limit at 0.08% BAC, with one exception: Utah uses a stricter 0.05% limit. These thresholds apply to adults 21 and older driving noncommercial vehicles. Commercial drivers face a lower federal limit of 0.04%, and drivers under 21 are typically held to zero-tolerance laws, where any detectable alcohol can result in a charge.
It’s worth noting that impairment begins well before you hit 0.08%. Reaction time, peripheral vision, and divided attention all measurably decline around 0.04% to 0.05%. The legal limit isn’t a safety threshold; it’s a line lawmakers drew for enforcement purposes.
How Fast Your BAC Rises and Falls
Your liver processes alcohol at a remarkably steady rate: roughly one standard drink per hour. A standard drink is 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of liquor. If you drink faster than your liver can keep up, the excess alcohol circulates through your bloodstream and your BAC climbs. Nothing speeds up this process. Coffee, cold showers, and food after drinking don’t help your liver work faster. Time is the only thing that brings your BAC back down.
What you eat before and during drinking, however, makes a significant difference in how high your BAC peaks. Drinking on an empty stomach allows alcohol to pass quickly into the small intestine, where it absorbs rapidly into the blood. A solid meal slows gastric emptying, which delays absorption and produces a lower, more gradual peak BAC. Solid food works better than liquid meals for this effect. Some research also suggests that eating increases the rate at which your body eliminates alcohol, giving you a double benefit: a lower peak and a faster return to zero.
Why the Same Drinks Hit People Differently
Two people can drink identical amounts and end up with very different BAC readings. The biggest factor is body water content, because alcohol distributes through body water. People with more body water dilute the same amount of alcohol across a larger volume, resulting in a lower BAC.
Women generally have a lower ratio of body water to body weight than men, which means the same number of drinks typically produces a higher BAC in women. This ratio also decreases as body fat percentage increases, regardless of sex, so body composition matters as much as body size. A 180-pound person with more muscle mass will generally have a lower BAC than a 180-pound person with more body fat after the same number of drinks.
Other variables include how quickly you drink, whether your stomach is empty, how much sleep you’ve gotten, and individual differences in liver enzyme activity. Medications that affect your liver can also change how efficiently you process alcohol.
How BAC Is Measured
There are two main ways to test BAC. A blood draw provides the most accurate reading and is the standard in medical and legal settings. A breathalyzer estimates BAC by measuring alcohol in your exhaled breath, converting that reading into an equivalent blood concentration. Breathalyzers are faster and more convenient, which is why police use them roadside, but blood tests are more precise. In legal cases, a blood draw typically carries more weight than a breath result.
Urine tests can also detect alcohol but are less reliable for pinpointing a current BAC, since urine reflects alcohol your body processed over a longer window rather than what’s circulating in your blood right now.
How Long Alcohol Stays Detectable
At one drink per hour, a person who has four drinks would need roughly four hours from their last drink for their BAC to return to 0.00%, assuming they spaced those drinks out. If all four were consumed in one hour, the math shifts: your liver still processes only one drink per hour, so you’d need closer to four or five hours after you stop drinking.
A practical example: if you stop drinking at midnight with a BAC around 0.12%, you’re looking at roughly four to five hours before reaching 0.00%. That means you could still be over the legal limit at 3 or 4 a.m., and potentially still impaired at breakfast. Many morning-after DUI arrests happen because people underestimate how long alcohol lingers in their system.