BAC stands for blood alcohol concentration, and it measures the amount of alcohol (ethanol) in your bloodstream. It’s expressed as a percentage: a BAC of 0.08% means there are 0.08 grams of alcohol per 100 milliliters of blood. This single number is what law enforcement, hospitals, and researchers use to quantify how intoxicated a person is at any given moment.
How BAC Is Measured
There are two main ways to determine BAC: a blood draw or a breath test. Blood testing is the most direct method, measuring the actual concentration of ethanol in a blood sample. Breath testing works indirectly, relying on the principle that alcohol in your blood evaporates into the air in your lungs at a predictable rate. Breathalyzers use a fixed ratio of 2,100 to 1, meaning the amount of alcohol in 2,100 milliliters of exhaled air equals the amount in 1 milliliter of blood.
Modern breathalyzers typically use one of two technologies. Fuel cell sensors contain platinum electrodes that generate an electrical current when alcohol molecules contact them. The stronger the current, the higher the reading. Infrared sensors work by passing light through a breath sample and measuring how much light the alcohol molecules absorb. Both approaches convert the breath reading into an estimated blood alcohol level using that 2,100:1 ratio.
What Different BAC Levels Feel Like
BAC isn’t just a legal number. It maps closely to specific physical and mental changes. The National Highway Traffic Safety Administration breaks it down at key thresholds:
- 0.02%: Slight relaxation, mild warmth, minor shifts in mood and judgment. Your ability to track moving objects and split your attention between two tasks starts to decline.
- 0.05%: Lowered alertness, reduced inhibition, and impaired judgment. Small-muscle control weakens, making it harder to focus your eyes. Steering and responding to sudden driving situations becomes noticeably more difficult.
- 0.08%: Poor muscle coordination affecting balance, speech, vision, and reaction time. Short-term memory, reasoning, and the ability to detect danger are all impaired. This is the legal limit for driving in 49 U.S. states.
- 0.15%: Significant loss of balance and muscle control. Vomiting is common unless tolerance is high. Driving ability is substantially impaired across the board.
Above 0.16%, the effects become more dangerous. Between 0.16% and 0.30%, you may have difficulty walking or speaking, experience blackouts, or lose consciousness. Above 0.31%, the risk of coma, respiratory failure, and death rises sharply.
How One Drink Affects Your BAC
For a person of average build, one standard drink raises BAC by roughly 0.01% to 0.03% over the course of an hour. A “standard drink” is 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of distilled spirits. That range is wide because the same amount of alcohol hits different people very differently.
Why BAC Varies Between People
Two people can drink the same amount and end up with very different BAC readings. Body size is the most obvious factor: a larger body has more blood volume to dilute the alcohol. But several other variables play a role.
Biological sex matters because body composition differs. People with a higher proportion of body fat and less body water (on average, women) will reach a higher BAC from the same number of drinks, since alcohol distributes through water, not fat. Genetics also influence how quickly your liver processes alcohol. The two main enzymes responsible for breaking down ethanol vary in efficiency from person to person based on inherited gene variants.
Food in your stomach slows alcohol absorption significantly. Drinking on an empty stomach allows ethanol to pass into the small intestine faster, where it’s absorbed into the bloodstream more quickly. Overall nutrition matters too. Severely restricting food intake to “save” calories for alcohol, sometimes called drunkorexia, can lead to dangerously rapid absorption and metabolic complications including a form of acidosis.
Certain medications can also raise BAC by interfering with alcohol absorption or metabolism, leading to higher peak levels than you’d otherwise reach from the same amount of drinking.
How Fast Your Body Clears Alcohol
Your liver eliminates alcohol at a relatively fixed rate, regardless of how much you’ve had. The average person lowers their BAC by about 0.01% to 0.02% per hour. That means someone at 0.10% would need between 5 and 10 hours to reach 0.00%. Coffee, cold showers, and exercise don’t speed this up. Only time works.
This rate explains why people can still be over the legal limit the morning after heavy drinking. If you stop drinking at midnight with a BAC of 0.15%, your body may not fully clear the alcohol until 8 to 15 hours later, well into the next day.
Legal BAC Limits
In the United States, the legal driving limit is 0.08% in every state except Utah, which lowered its threshold to 0.05% in 2019. For commercial drivers, the federal limit is 0.04%. For drivers under 21, most states enforce a zero-tolerance standard of 0.00% or close to it.
It’s worth noting that impairment begins well before 0.08%. At 0.05%, your ability to steer, track moving objects, and respond to emergencies on the road is already measurably reduced. The legal limit is a line drawn for enforcement, not the point at which alcohol starts affecting your driving.