What Is Blood Alcohol Concentration
Blood alcohol concentration (BAC) is the amount of alcohol circulating in the blood. NIAAA writes the binge threshold as 0.08%, or 0.08 grams of alcohol per deciliter. The same number appears in legal impaired-driving contexts in the United States for adults.
This page defines BAC and the public health thresholds tied to it. It does not provide drink-to-BAC calculators, personal dosing advice, or guidance on how to drink to a target level.
What BAC measures
NIAAA's Core Resource states that BAC is largely determined by how much and how quickly a person drinks, plus the body's rates of alcohol absorption, distribution, and metabolism. Alcohol passes from the stomach and intestines into the bloodstream and distributes through body water. Drinking on an empty stomach increases absorption rate and can produce a higher blood alcohol level than drinking with food. Alcohol is still absorbed faster than it is metabolized, so BAC rises when additional drinks arrive before prior drinks are cleared.
| Concept | Plain meaning |
|---|---|
| BAC 0.08% | 0.08 g alcohol per dL blood; NIAAA binge threshold |
| Absorption | Alcohol moving from gut into blood |
| Metabolism | Mainly liver enzymes breaking alcohol down over time |
| Distribution | Alcohol spreading through body water |
NIAAA's glossary also uses 0.08 g/dL when defining alcohol-impaired driving fatalities that involve a driver at that BAC or higher.
BAC and the binge drinking definition
Binge drinking is defined by NIAAA as a pattern that brings BAC to 0.08% or higher. For a typical adult, that often corresponds to four or more drinks for women or five or more drinks for men in about two hours. That drink count is a population proxy, not a personal prescription. Sex, body size, medications, food, and pace all change outcomes. This article will not convert your drinks into a predicted BAC.
For rates and harm context, see binge drinking statistics. For pattern questions without a diagnosis claim, see am I drinking too much.
What happens as BAC rises
NIAAA's alcohol overdose fact sheet states that as BAC increases, so do alcohol's effects and the risk of harm. Even small increases in BAC can decrease motor coordination, make a person feel sick, and cloud judgment, raising risk of injury from falls or car crashes, violence, and unprotected or unintended sex. At high BAC levels, blackouts, loss of consciousness, and death can occur.
Critical points from that fact sheet:
- BAC can continue to rise after a person stops drinking or is unconscious, because alcohol remaining in the stomach and intestine keeps entering the blood.
- An alcohol overdose occurs when so much alcohol is in the bloodstream that brain areas controlling breathing, heart rate, and temperature control begin to shut down.
- Symptoms of overdose include mental confusion, difficulty remaining conscious, vomiting, seizures, trouble breathing, slow heart rate, clammy skin, dulled responses such as no gag reflex, and extremely low body temperature.
If you suspect an alcohol overdose, call 911. Do not wait for every symptom to appear.
Factors that change BAC (without personal math)
NIAAA lists factors that tip the balance from impairment toward life-threatening overdose: age, sensitivity to alcohol (tolerance), sex, speed of drinking, medications, and amount of food eaten. Combining alcohol with opioids or sedative hypnotics increases overdose risk. Even over-the-counter antihistamines mixed with alcohol can be dangerous, according to the same fact sheet.
The Core Resource also notes that alcohol can impair driving even at amounts that do not produce a noticeable "buzz," and that high tolerance or a low level of response is a reason for caution because people with that trait tend to drink more.
BAC is not a wellness target
| Use of BAC language | Appropriate? |
|---|---|
| Understanding the 0.08% binge threshold | Yes, definitional |
| Reading public health or legal references | Yes, contextual |
| Planning drinks to hit a chosen BAC | No, not on this page |
| Guessing you are safe because you feel sober | No |
Feeling less intoxicated than someone else at a similar intake can reflect tolerance, body composition, or timing. It does not prove a safe BAC. For related vocabulary, see the sobriety glossary.
Dependence and stopping
People who drink heavily or daily should involve a clinician before stopping. Severe withdrawal symptoms such as seizures, hallucinations, or delirium tremens are medical emergencies. BAC definitions do not replace withdrawal assessment.
What this page will not do
- No drink-to-BAC calculators or formulas presented as advice
- No coaching on how to stay under a numeric BAC while drinking
- No detox or taper instructions
- No claims that any non-zero BAC carries zero risk
Practical takeaway
Blood alcohol concentration is a laboratory-style measure of how much alcohol is in the blood. The number that shows up most in US binge definitions and adult impaired-driving references is 0.08%. Knowing that threshold helps you read research and public health guidance. It is not a DIY intoxication dashboard.
Orlyn, which we make, is an iOS app with a live sober streak, daily check-ins, craving tools, and a 24/7 AI coach labeled as AI, not medical care. Use it as a complement to professional support if you are changing your drinking, not as a tool for managing BAC.
Frequently asked questions
What is blood alcohol concentration?
Blood alcohol concentration (BAC) is the amount of alcohol in the blood, often written as a percentage. NIAAA describes 0.08% as 0.08 grams of alcohol per deciliter of blood.
What BAC defines binge drinking?
NIAAA defines binge drinking as a pattern that brings BAC to 0.08% or higher. For a typical adult, that often corresponds to 4 or more drinks for women or 5 or more for men in about 2 hours.
Can BAC keep rising after someone stops drinking?
Yes. NIAAA states that BAC can continue to rise even when a person stops drinking or is unconscious, because alcohol still in the stomach and intestine continues to enter the bloodstream.
Does a lower buzz mean a safe BAC?
No. NIAAA notes that alcohol can impair driving even at amounts that do not produce a noticeable buzz, and that high tolerance is a reason for caution rather than reassurance.
What are signs of alcohol overdose?
NIAAA lists mental confusion, trouble staying conscious, vomiting, seizures, slow or irregular breathing, slow heart rate, clammy skin, dulled responses such as no gag reflex, and extremely low body temperature. Call 911 if you suspect overdose.