Blood alcohol level, also called blood alcohol concentration (BAC), is a measure of how much alcohol is currently in your bloodstream, expressed as a percentage. A BAC of 0.08% means that 0.08% of your blood by volume is alcohol. In the United States, 0.08% is the legal limit for driving, and even levels well below that threshold produce measurable impairment in judgment, coordination, and reaction time.
How BAC Is Measured
BAC is calculated in grams of alcohol per 100 milliliters of blood. Law enforcement typically estimates it through breath testing, which measures alcohol vapor in your lungs and converts it to a blood-equivalent reading. Blood draws give the most precise result and are often used in hospitals or as legal evidence.
Researchers and online calculators often use the Widmark equation to estimate BAC. The formula divides the total grams of alcohol consumed by your body weight (in grams) multiplied by a distribution ratio: 0.68 for men and 0.55 for women. That ratio reflects the fact that alcohol distributes through body water, and men typically carry a higher percentage of water relative to body fat. The result is then adjusted downward by about 0.015 to 0.016 percentage points for each hour since you started drinking, which accounts for your body steadily metabolizing the alcohol.
What Counts as One Drink
In the United States, a standard drink contains 0.6 ounces (14 grams) of pure alcohol. That translates to:
- Beer: 12 ounces at 5% alcohol
- Malt liquor: 8 ounces at 7% alcohol
- Wine: 5 ounces at 12% alcohol
- Liquor or spirits: 1.5 ounces (one shot) at 40% alcohol (80 proof)
These are smaller portions than many people expect. A typical restaurant pour of wine is often 6 to 8 ounces, and craft beers frequently run 7% to 10% alcohol, so a single glass or pint can easily equal one and a half or two standard drinks.
How Each BAC Level Feels
Impairment begins well before you feel “drunk.” The National Highway Traffic Safety Administration breaks down the effects at each stage:
At 0.02%, most people notice a slight sense of relaxation, body warmth, and a subtle shift in mood. Visual tracking slows, and your ability to do two things at once starts to decline. You may not feel impaired at all, but measurable changes in brain function are already underway.
At 0.05%, behavior becomes more exaggerated. Fine motor control slips, making it harder to focus your eyes precisely. Judgment is noticeably impaired, alertness drops, and inhibitions loosen. Behind the wheel, steering accuracy and the ability to respond to sudden hazards both decrease. This level is the legal threshold for a lesser charge (driving while ability impaired) in some states, including Colorado.
At 0.08%, the legal DUI limit, coordination deteriorates across the board: balance, speech, vision, reaction time, and hearing are all affected. Short-term memory, concentration, and the ability to process information suffer. Danger detection slows considerably.
At 0.10%, reaction time and physical control are clearly degraded. Speech becomes slurred, thinking slows, and maintaining a lane while driving becomes difficult.
At 0.15%, muscle control is far below normal, balance is seriously compromised, and vomiting may occur. At this level, a person has substantial impairment in virtually every function needed to operate a vehicle or navigate basic tasks safely.
When BAC Becomes Dangerous
As BAC climbs above 0.15%, the risks shift from impairment to medical emergency. High levels can cause blackouts (gaps in memory where you remain conscious but form no new memories) and loss of consciousness. At the most extreme levels, alcohol begins shutting down the brain regions that control breathing, heart rate, and body temperature. This is alcohol overdose, commonly called alcohol poisoning, and it can be fatal.
There is no single lethal BAC that applies to everyone. Tolerance, body composition, and how quickly alcohol was consumed all play a role. But the danger zone generally starts around 0.25% to 0.30%, where loss of consciousness and suppressed breathing become likely. Deaths have been recorded at a range of levels above that, particularly when a large amount of alcohol is consumed rapidly, as in binge drinking or drinking games.
What Determines Your BAC
Two people can drink the same amount and end up with very different blood alcohol levels. The biggest factors are body weight, biological sex, and body composition. A smaller person reaches a higher BAC from the same number of drinks because the alcohol is distributed through less total body water. Women typically reach higher BAC levels than men of the same weight because of differences in body water percentage and the distribution ratio used in the Widmark equation (0.55 for women versus 0.68 for men).
How fast you drink matters enormously. Your liver processes alcohol at a roughly fixed rate, so front-loading several drinks in a short window causes BAC to spike much higher than spacing the same drinks over several hours. Whether you’ve eaten also plays a role: food in your stomach slows the rate at which alcohol enters your bloodstream, blunting the peak BAC you reach.
Other contributing factors include your individual metabolism, medications that affect liver enzymes, and how much sleep you’ve had. Fatigue amplifies the cognitive effects of alcohol even if it doesn’t change the number on a breathalyzer.
How Fast Alcohol Leaves Your System
Your body eliminates alcohol at an average rate of about 0.010 to 0.020 percentage points per hour, with most people falling near 0.015 to 0.016 per hour. That rate is essentially fixed. Coffee, cold showers, and food do not speed it up.
In practical terms, someone who reaches a BAC of 0.10% would need roughly 5 to 10 hours to return to 0.00%. If you stop drinking at midnight with a BAC of 0.10%, you could still be above the legal limit at 6 a.m. This is why “morning after” DUI arrests are more common than many people realize.
Legal Limits in the U.S.
Every U.S. state sets the standard DUI limit at 0.08% BAC for drivers 21 and older. Some states also enforce a lower-tier offense. Colorado, for example, charges driving while ability impaired (DWAI) at 0.05%.
For drivers under 21, zero-tolerance laws apply. The threshold varies by state but is typically 0.02%, which can be reached with a single drink. Commercial vehicle operators, including truck and bus drivers, face a federal limit of 0.04%. Utah stands out as the only state that has lowered its standard DUI threshold to 0.05% for all adult drivers.