Blood alcohol content (BAC) is a measurement of how much alcohol is currently in your bloodstream, expressed as a percentage. A BAC of 0.08% means that for every deciliter of blood, there are 0.08 grams of pure alcohol. This number is the basis for drunk driving laws, medical assessments, and understanding how intoxicated a person actually is at any given moment.
How BAC Is Measured
BAC can be determined through a direct blood draw or estimated using a breath test. A blood test measures the actual concentration of ethanol in a blood sample and is the most accurate method. Breath testing, commonly done with a breathalyzer, works by measuring alcohol in the air you exhale and converting that to an estimated blood concentration. A study comparing over 400 paired breath and blood samples from drivers found that breathalyzers read lower than the actual blood result about 61% of the time, were within a close range 33% of the time, and read higher only 6% of the time. In other words, breathalyzers tend to slightly underestimate your true BAC.
What the Numbers Actually Mean
BAC is reported as a percentage, and even small-sounding numbers have real effects on your body and brain. Here’s what different levels generally look like:
- 0.02%: Slight warmth, relaxation, and mildly altered judgment. You may feel loosened up, but reaction time is already starting to slow.
- 0.05%: Lowered alertness, reduced coordination, and impaired ability to track moving objects. This is the legal driving limit in Utah.
- 0.08%: The legal limit in most U.S. states. At this point, muscle coordination, balance, reaction time, and judgment are all measurably impaired. This level also meets the clinical definition of binge drinking.
- 0.10–0.15%: Significant loss of motor control, slurred speech, and blurred vision. Maintaining balance while walking becomes difficult.
- 0.15–0.25%: Severe impairment. Blackouts (gaps in memory where you’re still conscious but forming no new memories) become likely. Vomiting and confusion are common.
- 0.25–0.35%: Risk of losing consciousness. The body’s basic functions, including breathing and heart rate, can become dangerously unstable.
- 0.35% and above: This range carries a serious risk of death. Breathing may slow to fewer than 8 breaths per minute or stop entirely.
Impairment doesn’t start at 0.08%. The CDC notes that impairment begins at lower BAC levels, which is why Utah set its limit at 0.05% and why zero-tolerance laws exist for underage drivers in every state.
What Counts as One Drink
A standard drink in the United States contains about 14 grams of pure alcohol. That translates to a 12-ounce beer at 5% alcohol, a 5-ounce glass of wine at 12%, or a 1.5-ounce shot of spirits at 40%. These are the serving sizes that BAC charts and calculators are built around. A pint of craft IPA at 7% or a generous pour of wine can easily count as one and a half or two standard drinks, which throws off any rough mental math you’re doing.
Why the Same Drinks Hit People Differently
Two people can drink identical amounts of alcohol and end up with very different BAC levels. The biggest factors are body composition, sex, weight, and whether you’ve eaten.
Alcohol dissolves in water, not fat. Your BAC is essentially determined by how much alcohol you consumed divided by how much water your body contains to dilute it. Someone who weighs more generally has more total body water, which dilutes the alcohol and produces a lower BAC. Someone smaller has less dilution capacity, so the same drink produces a higher concentration.
Sex plays a significant role beyond just body size. Women typically carry proportionally more body fat and less body water than men of the same weight. This means the alcohol they drink is distributed through a smaller volume of water, leading to higher peak BAC from the same dose. Research has shown that when alcohol doses are calculated based on total body water rather than body weight, the difference between men and women largely disappears. Women may also process alcohol less efficiently in the stomach and liver before it enters the bloodstream, allowing more of each drink to reach full circulation.
Food matters too. Eating before or while drinking slows the rate at which alcohol passes from your stomach into your small intestine, where most absorption happens. A slower absorption rate gives your liver more time to break down alcohol before it accumulates in your blood, resulting in a noticeably lower peak BAC. Drinking on an empty stomach does the opposite: alcohol moves quickly into the bloodstream, and your BAC spikes faster and higher.
How Your Body Eliminates Alcohol
Your liver does the heavy lifting. Once your BAC peaks, it drops at a fairly fixed rate of about 0.015% per hour. This rate doesn’t change much regardless of what you do. Coffee, cold showers, and exercise don’t speed it up.
A rough rule of thumb is that the liver can handle about one standard drink per hour, but this varies depending on your size, sex, and individual metabolism. If you reach a BAC of 0.08%, it will take roughly five to six hours for your body to return to 0.00%. A BAC of 0.15% could take ten hours or more. This is why people can still be legally impaired the morning after heavy drinking.
BAC and Alcohol Poisoning
At high BAC levels, alcohol suppresses the parts of your brain that control basic survival functions like breathing and consciousness. Warning signs of alcohol overdose include breathing that drops below 8 breaths per minute, gaps of 10 seconds or more between breaths, vomiting while unconscious, and skin that looks pale or bluish. Loss of consciousness at a high BAC is not “sleeping it off.” The body continues absorbing any alcohol still in the stomach, so BAC can keep rising even after someone passes out.