Blood alcohol level, also called blood alcohol concentration (BAC), is the weight of alcohol present in a specific volume of your blood. It’s expressed as a percentage: a BAC of 0.08% means there are 0.08 grams of alcohol per deciliter of blood. That number is the legal cutoff for impaired driving in 49 U.S. states, so it’s the figure most people encounter first, but the full BAC scale runs from 0.00% up past 0.40%, where death becomes a real possibility.
How BAC Is Measured
There are two main ways to measure blood alcohol level. A direct blood draw gives the most accurate result, reported either as a percentage (0.08% BAC) or in grams per milliliter (0.08 g/100 mL). The second method is a breathalyzer, which estimates BAC by measuring the alcohol in air exhaled from your lungs. Alcohol in your bloodstream passes into the tiny air sacs in your lungs at a predictable ratio, and the device uses that ratio to calculate a BAC equivalent.
Breathalyzers are convenient and fast, and clinical studies show a high correlation (r = 0.91) with blood test results. They’re accurate to within ±0.01% at lower concentrations. That said, breathing patterns, mouth temperature, and residual alcohol in the mouth from a recent sip or even mouthwash can throw the reading off. That’s why law enforcement often waits 15 to 20 minutes before administering a breath test, and why a blood draw remains the gold standard for legal proceedings.
What a Standard Drink Actually Contains
In the United States, one “standard drink” contains about 14 grams (roughly 0.6 fluid ounces) of pure alcohol. That translates to 12 ounces of regular beer at about 5% alcohol, 5 ounces of wine at about 12%, or 1.5 ounces of distilled spirits at about 40%. These are useful benchmarks, but real-world pours often exceed them. A generous glass of wine at a restaurant can easily be 7 or 8 ounces, which is closer to one and a half standard drinks. Knowing the actual alcohol content of what you’re drinking matters more than counting “drinks” by the glass.
What Happens at Each BAC Range
BAC effects follow a roughly predictable curve, though individual responses vary. Here’s what to expect at key thresholds:
- 0.02% to 0.03%: A mild sense of relaxation and slight warmth. Judgment begins to shift, but most people feel essentially normal. This is roughly where one drink lands a person of average weight.
- 0.04% to 0.06%: Lowered inhibitions, a feeling of mild euphoria, and slight impairment in reasoning and memory. Fine motor skills start to decline, and reaction time slows.
- 0.07% to 0.09%: Noticeable impairment in balance, speech, vision, and reaction time. Self-control and judgment are clearly reduced. This range straddles the legal driving limit in most states.
- 0.10% to 0.15%: Significant impairment of motor control. Speech becomes slurred, balance is poor, and reaction time is substantially delayed. Nausea and vomiting are common at the higher end.
- 0.16% to 0.24%: Disorientation, confusion, and emotional instability. Nausea is likely, and the risk of choking on vomit increases. Most people in this range need help walking.
- 0.25% to 0.34%: Severe impairment of all mental and physical functions. Loss of consciousness is common. The body’s gag reflex may stop working, which makes choking on vomit a serious danger.
- 0.35% and above: This is the range associated with coma and life-threatening respiratory depression. A BAC over 0.40% is frequently fatal, though individuals with very high tolerance have survived higher levels.
An important point about tolerance: people who drink heavily over time can develop the ability to appear relatively functional at BAC levels that would incapacitate others. This is the body adapting to a toxic substance, not a sign that alcohol is causing less damage. A person with high tolerance is still impaired in reaction time and judgment, even if they don’t seem like it.
Factors That Raise or Lower Your BAC
Two people can drink the same amount and end up at very different BAC levels. Several factors explain why.
Body composition and sex. Women typically reach a higher BAC than men after consuming the same amount of alcohol, even at similar body weights. Women tend to have a higher proportion of body fat and lower proportion of body water. Because alcohol dissolves in water rather than fat, there’s less tissue to absorb it, so more stays concentrated in the bloodstream. Men also produce more of the stomach enzymes that begin breaking down alcohol before it reaches the blood.
Food in the stomach. Eating before or while drinking slows intoxication significantly. Food triggers a valve at the bottom of the stomach to close during digestion, which keeps alcohol from quickly passing into the small intestine, where most absorption happens. Greasy, fatty, and high-protein foods work best because they take longer to digest: think pizza, cheese, fried foods, or anything with a good amount of fat.
Speed of drinking. Gulping drinks or doing shots produces a faster, higher BAC spike than sipping the same amount over a longer period. Your liver processes alcohol at a relatively fixed rate, so the faster you pour it in, the more stacks up in your blood waiting to be metabolized.
Fatigue and stress. Physical, mental, or emotional exhaustion intensifies alcohol’s effects. If you’re running on little sleep or under heavy stress, the same number of drinks will hit harder than they would when you’re well-rested.
Mood. Alcohol amplifies whatever emotional state you’re already in. Someone who starts drinking while feeling low is more likely to become severely depressed, not less.
How Your Body Processes Alcohol
Your liver does the heavy lifting. It breaks down alcohol at a roughly constant rate regardless of how much you’ve consumed. For most people, that rate works out to about one standard drink per hour, which translates to a BAC reduction of roughly 0.015% to 0.017% per hour. That rate doesn’t speed up with coffee, cold showers, or exercise. Those might make you feel more alert, but your BAC stays the same until your liver works through it.
This fixed processing rate is why binge drinking is so dangerous. If you consume four drinks in an hour, your liver can only handle one of them in that time. The other three stack up, and your BAC keeps climbing even after you stop drinking. Peak BAC often arrives 30 to 90 minutes after the last drink, depending on stomach contents and how quickly you were drinking.
Legal Limits for Driving
In the United States, 49 states set the legal per se limit at 0.08%. Utah is the exception, having lowered its limit to 0.05% in December 2018. All states set the limit at 0.02% or lower for drivers under 21. Some states impose consequences even below the standard threshold: Colorado and New York have “driving while ability impaired” laws that cover impairment at BAC levels below 0.08%, and West Virginia can revoke a license above 0.05%.
Internationally, many countries use a stricter standard. Scotland lowered its limit from 0.08% to 0.05% in 2014, while England, Wales, and Northern Ireland kept theirs at 0.08%. Most of continental Europe, Australia, and much of Asia use 0.05% or lower. Some countries, including Brazil and Saudi Arabia, enforce a zero-tolerance policy.
It’s worth noting that measurable impairment in reaction time and divided attention begins well below 0.08%. Studies consistently show meaningful declines in driving-related skills starting around 0.02% to 0.04%. The legal limit is not a safety threshold; it’s a legal one.
Estimating BAC With the Widmark Formula
If you’ve seen online BAC calculators, most of them use some version of a formula developed in the 1930s by Erik Widmark. The basic equation is: BAC = grams of alcohol consumed divided by (body weight in kilograms multiplied by a distribution factor). The distribution factor accounts for how much of your body is water versus fat. It’s typically set at 0.7 for men and 0.6 for women of normal weight, with adjustments for body composition. Lower values are used for people with more body fat, higher values for leaner individuals.
These calculators give a rough estimate, but the margin of error is significant. One analysis found the estimate can vary by about 42% from the actual measured BAC. That’s a huge range. A calculator might tell you you’re at 0.05% when your actual level is 0.07%. These tools are useful for general awareness, but they shouldn’t be used to decide whether you’re safe to drive.