BAC measures the amount of alcohol in your blood, expressed as a percentage. A BAC of 0.08%, for example, means there are 0.08 grams of alcohol per 100 milliliters of blood. This single number is used to estimate how impaired a person is, set legal driving limits, and guide medical decisions during alcohol-related emergencies.
How BAC Is Calculated
BAC stands for blood alcohol content (sometimes called blood alcohol concentration or blood alcohol level). The result can be reported as a percentage, like 0.05% BAC, or in grams per milliliter, like 0.05 g/100 mL. Both formats mean the same thing.
The number reflects how much ethanol has made it into your bloodstream relative to the volume of your blood. After you swallow a drink, the alcohol is absorbed primarily through the lining of the small intestine into the veins that feed the liver. From there it diffuses across cell membranes and distributes through nearly all tissues in the body. Your BAC rises as absorption outpaces your liver’s ability to break the alcohol down, then gradually falls as your liver catches up.
What Affects Your BAC
Two people can drink the same amount and end up with very different BAC levels. The main variables are body weight, body composition, biological sex, how much food is in the stomach, and how fast the drinks are consumed. These factors are captured in a formula forensic scientists have used for decades, where BAC equals the mass of alcohol consumed divided by the person’s weight multiplied by a body-water constant, minus the amount eliminated over time.
That body-water constant is lower for women than for men at the same body mass index, which means women generally reach a higher BAC from the same amount of alcohol. People with a higher BMI also tend to have a lower constant, shifting the math further. Drinking on an empty stomach speeds absorption and produces a faster, higher peak. Drinking with food slows things down because the stomach takes longer to empty its contents into the small intestine where most absorption happens.
How Your Body Clears Alcohol
Your liver does the heavy lifting, breaking down ethanol through enzymes at a roughly predictable pace. On an empty stomach, most people eliminate alcohol at a rate of 0.010 to 0.015 percentage points per hour. After eating, the rate tends to be slightly faster, around 0.015 to 0.020 per hour. Heavy, chronic drinkers can clear alcohol even faster, at rates up to 0.035 per hour, because their liver enzymes have ramped up activity in response to repeated exposure.
For a moderate drinker, 0.015 per hour is a reasonable average. That means if your BAC peaks at 0.08%, it would take roughly five to six hours for your body to return to zero, assuming you’ve stopped drinking. No amount of coffee, water, or food speeds this process once the alcohol is already in your blood.
How BAC Is Measured
There are two main approaches: a direct blood draw or a breath test.
A blood test is the most accurate method. A sample is taken from a vein and analyzed in a lab to determine the exact concentration of ethanol. This is the gold standard used in hospitals and sometimes in legal proceedings.
Breathalyzers are far more common in roadside stops. They work on a simple principle: some of the alcohol in your blood crosses into the air in your lungs when you exhale. The device measures the alcohol in that breath sample and multiplies it by a fixed ratio to estimate your blood level. In the United States, Australia, and Canada, that ratio is 2,100 to 1, meaning the device assumes 2,100 liters of breath contain the same amount of alcohol as 1 liter of blood. Other countries use different ratios. The UK uses 2,300 to 1, and some Scandinavian and European nations use 2,000 to 1 or 2,400 to 1.
Because everyone’s actual breath-to-blood ratio varies slightly, breathalyzers aren’t perfectly precise. A study of over 400 paired breath and blood samples from Wisconsin drivers found that breathalyzer readings came in lower than the blood test result by more than 0.01 about 61% of the time, matched within 0.01 about 33% of the time, and read higher than the blood result only 6% of the time. In practice, breathalyzers tend to underestimate rather than overestimate your true BAC.
What Different BAC Levels Feel Like
BAC doesn’t just tell you how much alcohol is in your blood. It correlates with specific, measurable impairments. The National Highway Traffic Safety Administration breaks it down like this:
- 0.02%: Slight warmth, mild relaxation, some loss of judgment. Your ability to track moving objects and divide your attention between two tasks begins to decline. Most people wouldn’t feel noticeably “drunk” at this level.
- 0.05%: Lowered alertness, reduced coordination, and exaggerated behavior. Small-muscle control starts to slip, making it harder to focus your eyes. Steering a car becomes measurably more difficult, and your ability to respond to sudden hazards drops.
- 0.08%: This is the legal limit for driving in most U.S. states. Balance, speech, vision, and reaction time are all noticeably impaired. Short-term memory, concentration, and the ability to process information (like reading signs or detecting danger) are significantly reduced.
Beyond 0.08%, impairment accelerates. Levels above 0.20% can cause confusion and disorientation, and levels above 0.30% carry serious risk of loss of consciousness and life-threatening alcohol poisoning.
Legal BAC Limits Around the World
The legal limit for driving is not universal. In most of the United States, the threshold is 0.08%, though Utah has set its limit at 0.05%. Drivers under 21 face near-zero tolerance in most states, with limits at 0.02% or lower. Commercial drivers holding a federal license are held to 0.04%.
Most of Europe, along with Australia, has adopted 0.05% as the standard limit. Countries like Sweden, Poland, and Estonia go further, setting their limits at 0.02%. A handful of countries, including Czechia, Hungary, Romania, and Slovakia, enforce zero tolerance, meaning any detectable alcohol can result in penalties. England, Wales, and Northern Ireland remain at 0.08%, while Scotland lowered its limit to 0.05% in 2014.
These differences reflect varying cultural attitudes toward alcohol and road safety, but the underlying measurement, BAC, is the same everywhere. Whether tested by blood draw in a hospital or a roadside breath device, the number represents the same thing: grams of alcohol per 100 milliliters of your blood.