BAC stands for blood alcohol concentration (sometimes called blood alcohol content or blood alcohol level). It’s a measurement of how much alcohol is currently in your bloodstream, expressed as a percentage. A BAC of 0.08%, for example, means that 0.08 grams of alcohol are present in every 100 milliliters of your blood. That number is also the legal limit for driving in 49 U.S. states, with Utah setting a stricter cutoff at 0.05%.
How BAC Is Measured
There are two main ways to measure BAC: a blood draw or a breath test. A blood test is the more direct method, measuring the actual concentration of alcohol in a blood sample. Breath tests (commonly called breathalyzers) estimate BAC by measuring alcohol vapor in exhaled air and converting it to an equivalent blood level.
Breathalyzers tend to read slightly lower than blood tests. In a study of over 400 paired breath and blood samples collected from drivers within an hour of each other, the breathalyzer result came in lower than the blood result 61% of the time. It matched closely (within 0.01%) about a third of the time, and read higher only 6% of the time. This is one reason law enforcement sometimes follows up a roadside breath test with a blood draw.
What Different BAC Levels Feel Like
The effects of alcohol follow a fairly predictable pattern as BAC rises, though individual tolerance plays a role. The National Highway Traffic Safety Administration breaks it down this way:
At 0.02%, which is roughly one drink for many people, you’ll feel slight warmth and relaxation. Mood shifts a little, judgment dips slightly, and your ability to track a moving object starts to decline. Most people wouldn’t describe themselves as “drunk” at this level.
At 0.05%, behavior becomes more exaggerated and inhibitions loosen. You may have trouble focusing your eyes, and alertness drops noticeably. Coordination suffers enough that steering a car becomes harder, and your ability to respond to sudden driving situations is reduced.
At 0.08%, the legal limit in most states, impairment is significant across the board. Balance, speech, vision, reaction time, and hearing all deteriorate. Judgment, self-control, reasoning, and memory are all compromised. Short-term memory loss is common, and your brain processes information more slowly, making it harder to detect danger.
At 0.15%, muscle control is far below normal and balance is seriously impaired. Vomiting is likely unless the person reached that level very gradually or has built up a high tolerance. At this point, the ability to safely control a vehicle is essentially gone.
Above 0.30%, the risk of losing consciousness, experiencing alcohol poisoning, or dying becomes very real. Breathing can slow dangerously, and the body’s protective reflexes (like gagging) may stop working.
Factors That Raise or Lower Your BAC
Two people can drink the same amount and end up with very different BAC readings. Several biological factors explain why:
- Body size: Larger people generally reach a lower BAC from the same number of drinks because alcohol is distributed across more body mass.
- Biological sex: Women typically reach a higher BAC than men after the same amount of alcohol, even at the same body weight. This is largely because women tend to have a higher proportion of body fat and less total body water, so alcohol is more concentrated in the bloodstream.
- Food in the stomach: Drinking on an empty stomach allows alcohol to absorb into the bloodstream much faster, producing a higher and quicker peak BAC. A full meal, especially one with fat and protein, slows absorption considerably.
- Speed of drinking: Consuming several drinks in a short window raises BAC faster than your body can process the alcohol, leading to a sharper spike.
- Metabolism: People with faster metabolic rates clear alcohol from their blood more quickly, keeping their BAC lower over time.
How Fast Your Body Clears Alcohol
The average person metabolizes about 7 grams of alcohol per hour, which is roughly equivalent to one standard drink per hour. A standard drink is 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of distilled spirits. If you’re drinking faster than that pace, your BAC is rising.
This rate isn’t perfectly fixed. It varies from person to person and can even shift depending on how much alcohol is in your system at a given moment. But as a general rule, you can expect your BAC to drop by roughly 0.015% per hour once you stop drinking. So if you finish your last drink at midnight with a BAC of 0.08%, you wouldn’t be back to zero until roughly 5:00 a.m. Coffee, cold showers, and food do not speed up this process. Only time does.
Legal BAC Limits
In all 50 U.S. states, Washington D.C., and Puerto Rico, it is illegal to drive with a BAC of 0.08% or higher. Utah is the one exception, having lowered its limit to 0.05% in 2018. Commercial drivers (those operating semi-trucks, buses, and similar vehicles) are held to a stricter federal standard of 0.04%. Drivers under the age of 21 face zero-tolerance laws in every state, meaning any detectable amount of alcohol can result in penalties.
It’s worth noting that you can still be charged with impaired driving below the legal limit if an officer determines your ability to drive is compromised. The 0.08% threshold is a “per se” limit, meaning you’re legally intoxicated at that number regardless of how you feel or behave. But impairment starts well before that line, as the effects at 0.05% make clear.