What blood alcohol concentration measures
Blood alcohol concentration is the proportion of your bloodstream that is ethanol, written as a percentage by volume. A BAC of 0.08% means eight hundredths of a gram of alcohol per hundred milliliters of blood. It is the number breathalysers estimate, the number blood draws measure, and the number written into every drink-driving statute in the United States.
What it is not is a measure of how drunk you feel. Tolerance changes the feeling enormously and changes the blood chemistry not at all. Two people at the identical BAC can present completely differently, and the one who seems fine is just as impaired where it counts — reaction time, judgement, divided attention.
The Widmark equation, in full
This calculator uses the Widmark equation, published by Erik Widmark in the 1930s and still the basis of forensic BAC estimation:
A = fluid ounces of pure alcohol consumed
W = body weight in pounds
r = 0.73 for men, 0.66 for women
H = hours since the first drink
The ounces of pure alcohol come from the drinks themselves: volume multiplied by the alcohol by volume percentage. Three 12-ounce beers at 5% is 36 ounces of liquid carrying 1.8 ounces of pure alcohol.
The r term is the Widmark factor — the fraction of body mass that behaves like water and therefore dilutes alcohol. Alcohol distributes into body water, not fat, which is why the factor differs by sex on average: women carry proportionally more body fat and less water, so the same drink produces a higher concentration.
Take the example the form opens on: three 12-ounce beers at 5% is 1.8 ounces of alcohol, for a 168-pound man, three and a half hours in. That gives (1.8 × 5.24) ÷ (168 × 0.73) = 0.0769, minus 0.015 × 3.5 = 0.0525, leaving roughly 0.024%.
What counts as one drink
A US standard drink is defined by its alcohol content, not its glass. Each of these contains about 0.6 fluid ounces of pure alcohol:
- 12 ounces of regular beer at about 5%
- 5 ounces of table wine at about 12%
- 1.5 ounces of 80-proof spirits at 40%
This is where most self-estimates go wrong. A craft IPA at 7.5% in a 16-ounce pour is two standard drinks, not one. A generous home measure of spirits is routinely double a bar shot. A large glass of wine at a restaurant is often 8 or 9 ounces. Enter what was actually in the glass rather than counting glasses, which is why the calculator asks for size and strength separately.
Why only time sobers you up
Alcohol leaves the body almost entirely through the liver, which oxidizes it at a nearly constant rate regardless of how much is present. That is the 0.015 per hour in the formula: about one standard drink cleared per hour for most adults, give or take.
Because the rate is fixed, nothing speeds it up. Coffee, cold showers, food, exercise, water, sleep — none of them change how fast the liver works. Caffeine is the dangerous one, because it masks drowsiness without touching impairment, producing a person who feels alert and is not.
Food does matter, but earlier in the process: eating before or while drinking slows absorption from the stomach, which lowers the peak. It does not reduce the total. It also means BAC often keeps climbing for a while after the last drink, so a reading taken as you leave can be lower than the one an hour later.
Legal limits in the United States
The limit for drivers aged 21 and over is 0.08% in every state except Utah, which set 0.05% in 2018. For drivers under 21, zero-tolerance laws apply everywhere, with thresholds typically between 0.00% and 0.02%. Commercial drivers are held to 0.04%.
Two things are widely misunderstood. First, being under the limit is not a defence — every state allows an impaired-driving charge below 0.08% if driving is affected. Second, impairment starts well below any legal threshold: measurable effects on divided attention and reaction time appear from around 0.02%, long before anybody feels drunk.
What different levels do to a person
The impairment bands shown in the result panel come from the standard toxicology literature. Broadly: subtle effects detectable only by testing below 0.03%; mild euphoria and reduced inhibition to 0.06%; measurable loss of reasoning, depth perception and peripheral vision to 0.10%; loss of motor control and slurred speech to 0.20%; vomiting and blackouts beyond that; and from 0.30% upwards, genuine risk of respiratory depression, coma and death.
Alcohol poisoning is a medical emergency, not something to sleep off. Confusion, vomiting while unresponsive, seizures, slow or irregular breathing, blue-tinged or pale skin, or unconsciousness that cannot be roused all warrant an ambulance immediately. Never leave someone in that state alone to sleep, and never assume that lying down is enough to keep an unconscious person’s airway clear.
Why this number can be badly wrong for you
Widmark is a population average fitted in the 1930s. For any individual it can be out by a wide margin in either direction:
- Body composition, not just weight. Two people at the same weight with different fat percentages have different amounts of body water, and the sex-based r value is a crude stand-in for that.
- Absorption is not instant. The equation assumes all the alcohol is already in your blood. Drinking over an evening, on an empty or full stomach, changes the curve substantially.
- Elimination varies. The 0.015 per hour figure ranges roughly from 0.012 to 0.020 between individuals, and is affected by liver health, medication and regular heavy drinking.
- Medication and health conditions can change both absorption and clearance, sometimes dramatically.
- It cannot know what you actually drank. Free pours, unclear ABV and topped-up glasses all mean the input is a guess.
An error of 0.02% either way is entirely plausible, which is the whole distance between the legal limit and a charge. That is why this page cannot tell you whether you may drive, and does not try.
If drinking has stopped being a choice
If you have found yourself checking a calculator like this often, or drinking more than you meant to, or unable to cut down when you decide to, that is worth taking seriously and it is common. In the United States, SAMHSA’s National Helpline is free, confidential and staffed 24 hours a day, every day, on 1-800-662-4357. It provides treatment referral and information, and you do not need insurance to call.
Frequently asked questions
How is blood alcohol concentration calculated?
With the Widmark equation: ounces of pure alcohol times 5.24, divided by body weight in pounds times a distribution factor of 0.73 for men or 0.66 for women, then minus 0.015 for every hour since the first drink.
How long does it take for BAC to reach zero?
Divide the current level by 0.015 and round up. A BAC of 0.06% takes about four hours to clear. The rate is set by the liver and cannot be increased.
Does coffee or a cold shower lower BAC?
No. Neither affects how fast the liver processes alcohol. Caffeine is actively risky because it masks tiredness without reducing impairment, so a person feels more capable than they are.
Can this calculator tell me if I am safe to drive?
No, and it should not be used that way. Individual variation, absorption timing and uncertainty about what was actually poured mean the estimate can easily be out by enough to cross a legal limit. If you have been drinking, do not drive.
Why do men and women get different results?
Alcohol distributes into body water rather than fat. Women on average carry proportionally more body fat, so the same amount of alcohol is diluted into less water and produces a higher concentration.
What is a standard drink?
In the United States, any drink containing about 0.6 fluid ounces of pure alcohol: roughly a 12-ounce beer at 5%, a 5-ounce glass of wine at 12%, or a 1.5-ounce shot of 40% spirits.
Why does my BAC keep rising after I stop drinking?
Because alcohol already in the stomach and small intestine is still being absorbed. Peak concentration typically arrives somewhere between 30 and 90 minutes after the last drink, later if you have eaten.
Is a BAC of 0.08% the limit everywhere in the US?
For drivers 21 and over it is the limit in every state except Utah, where it is 0.05%. Commercial drivers are limited to 0.04%, and under-21 drivers face zero-tolerance thresholds near 0.00%.
This calculator gives a rough statistical estimate for general education only. It is not medical or legal advice, it cannot measure your actual blood alcohol concentration, and it must never be used to decide whether you are fit or legal to drive, operate machinery, or supervise anyone. Individual results vary widely with body composition, food, medication, health and drinking pattern. If you are worried about your own or someone else’s drinking, speak to a doctor, or in the US call SAMHSA’s free 24/7 National Helpline on 1-800-662-4357. If someone may have alcohol poisoning, call emergency services immediately. Learn more about how we build these tools or read our privacy policy.