BAC Calculator

Blood alcohol concentration is the standard measure of alcohol intoxication, written as a percentage of alcohol by volume of blood. In the United States a BAC of 0.08% means 0.08 grams of alcohol in every 100 mL of blood.

This calculator estimates BAC from what you drank, your body weight, your sex and how long ago you started. It is an estimate built on population averages, and the section on accuracy below explains why it should never be used to decide whether you are fit to drive.

How BAC is calculated

The Widmark formula, published by Erik Widmark in 1932, still underpins almost every BAC calculation in use.

BAC% = (A / (W x r)) x 100 - (0.015 x H)

where:

• A is the mass of pure alcohol consumed, in grams
• W is body weight in grams
• r is the Widmark factor, the fraction of the body that alcohol distributes into: about 0.67 for men and 0.60 for women
• 0.015 is the percentage points of BAC the liver clears per hour
• H is hours since the first drink

Alcohol dissolves in water, not fat, so r is effectively the share of your body that is water. Women average a higher body fat percentage than men at the same weight, which leaves less water for the same alcohol to spread through, and produces a higher BAC from an identical drink.

A worked example

Two 330 mL beers at 5% ABV contain 33 mL of pure alcohol. Ethanol weighs 0.789 grams per millilitre, so that is 26 grams of alcohol.

For a 70 kg man, r is 0.667, giving 26 / (70,000 x 0.667) x 100, which is a peak BAC of 0.056%. Two hours later the liver has cleared 0.03 percentage points, leaving 0.026%, and it takes roughly two more hours to reach zero.

The same drinks in a 70 kg woman produce 0.032% at the two-hour mark, because r is 0.60 rather than 0.667. The same two beers in a 100 kg man leave 0.0091%. Nothing changed except the body they went into.

Legal limits

The US limit for drivers over 21 is 0.08% in every state except Utah, which moved to 0.05% in 2018. Commercial drivers are held to 0.04% under federal rules. Drivers under 21 face zero-tolerance laws in all fifty states, with limits between 0.00% and 0.02% depending on the state.

Most of Europe sets 0.05%. Sweden, Norway and Poland use 0.02%. A number of countries, including the Czech Republic, Hungary and Romania, allow nothing at all. Limits are lower for newly qualified drivers in many places, and penalties in most jurisdictions escalate sharply above roughly twice the limit.

Being under the limit is not the same as being safe to drive. Measurable impairment of reaction time, tracking and divided attention appears well below 0.08%, which is why the National Transportation Safety Board has recommended 0.05% for the US since 2013.

Standard drinks

A US standard drink contains 14 grams of pure alcohol, which is roughly 12 fl oz of 5% beer, 5 fl oz of 12% wine, or 1.5 fl oz of 40% spirits. Each of those delivers about the same alcohol despite very different volumes.

Standard measures differ by country, which matters if you are reading foreign guidance. A UK unit is 8 grams, an Australian standard drink is 10 grams, and a Japanese unit is 20 grams. The same "two drinks a day" recommendation means two and a half times as much alcohol in Japan as in the UK.

Real servings rarely match the standard. A pint of 5.5% craft beer is nearly two standard drinks. A generous pour of wine at home is often 8 oz rather than 5. Cocktails routinely carry two or three shots. Counting glasses undercounts alcohol.

Why the estimate can be wrong

Widmark describes an average body. Yours is not average, and several things push the real figure away from the calculated one.

Food. Eating before or while drinking slows gastric emptying, so alcohol reaches the bloodstream more gradually. A full stomach can cut peak BAC substantially compared with the same drinks taken on an empty one. It does not reduce the total alcohol absorbed, only the height of the peak.

Sex differences beyond body water. Women produce less gastric alcohol dehydrogenase, the enzyme that breaks down some alcohol in the stomach wall before it reaches the blood, so proportionally more of each drink gets through.

Genetics. Around a third of people of East Asian descent carry an ALDH2 variant that slows the breakdown of acetaldehyde, causing facial flushing, nausea and a raised heart rate at low doses. Elimination rates vary between individuals from roughly 0.012% to 0.020% per hour, and regular heavy drinkers clear alcohol faster through enzyme induction while showing the same impairment.

Body composition. The formula uses total weight, not lean mass. Two people at 80 kg with very different body fat percentages will not reach the same BAC.

Timing and carbonation. Four drinks over four hours produce a much lower peak than four drinks in one. Carbonated mixers speed absorption measurably.

Medication and health. Liver disease, some antibiotics, antihistamines, sedatives and several other drug classes all alter how alcohol is handled or how impaired you become at a given BAC.

Because of all this, a Widmark estimate can sit well either side of a real reading. Only a breath or blood test measures BAC.

What does not sober you up

Nothing accelerates the liver. It clears alcohol at a roughly fixed rate regardless of what else you do, and that is the single most useful fact on this page.

Coffee does not lower BAC. Caffeine makes a drunk person an alert drunk person, which research suggests is more dangerous rather than less, because the subjective sense of impairment fades while the actual impairment does not. Cold showers, exercise, fresh air, energy drinks, vomiting and eating after the fact do nothing to the number either. Sleep does not speed it up, though time passing during sleep does. Someone who stops drinking at 0.20% will still be over the US driving limit around eight hours later, which is why morning-after driving accounts for a meaningful share of drink-driving arrests.

Alcohol and health

Short-term effects include dehydration and intoxication. Sustained use changes liver and brain metabolism, and can lead to alcohol dependence, meaning use that causes physical or mental health problems.

The long-standing idea that a daily glass of red wine is good for you has not held up. The 2018 Global Burden of Disease analysis in The Lancet, covering 195 countries, concluded that the level of consumption minimising harm across all causes is zero, and the World Health Organization stated in 2023 that no amount of alcohol is safe for health. Earlier studies suggesting cardiovascular benefit are now thought to have been distorted by comparing drinkers against a group of non-drinkers that included people who had stopped because they were already unwell.

Some potential benefits of light drinking have been reported, including a lower risk of heart disease, ischaemic stroke and diabetes. These are potential rather than guaranteed, they do not apply to everyone, and they sit alongside a raised risk of several cancers even at light intake. Alcohol is a Group 1 carcinogen, and the association with breast cancer appears at levels well below heavy drinking. Nobody should start drinking for health reasons.

Heavy drinking has no upside. The Mayo Clinic defines it as more than three drinks a day for women and for men over 65, and more than four a day or fourteen a week for men under 65. At that level the risks include several cancers, heart muscle damage leading to failure, stroke, high blood pressure, liver disease, brain damage, serious accidental injury and sudden death in people with existing cardiovascular disease.

If you already drink lightly and are healthy, continuing is a reasonable choice to make with your doctor, who can weigh your own history and medication against it.

Effects at each level

Impairment tracks BAC closely, from effects at 0.001% to 0.029% that only show up in laboratory testing, through the loss of concentration, reasoning, reflexes and motor control that follows, to a serious risk of death above 0.40%. The table under the calculator sets out behaviour and impairment at each band.

Alcohol poisoning is a medical emergency. Confusion, vomiting, seizures, slow or irregular breathing, cold or blue-tinged skin, and being unable to wake someone all warrant an ambulance. Someone unconscious should never be left to sleep it off, since BAC can keep climbing for up to an hour after the last drink as alcohol already in the stomach continues to be absorbed.