BAC Calculator
Estimate blood alcohol concentration with the Widmark formula from drinks, body weight, and time — and understand why the number should never decide whether you drive.
Comment utiliser cette calculatrice
- 1Enter your sex and body weight — both change how alcohol distributes.
- 2Count your drinks in US standard units: 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of 40% spirits each count as one.
- 3Enter how many hours have passed since your first drink.
- 4Treat the number as a rough illustration of how slowly alcohol leaves the body — never as clearance to drive.
Comment ça marche
The Widmark formula
BAC = (A ÷ (W × r)) × 100 − (β × t) A = grams of pure alcohol (standard drinks × 14 g) W = body weight in grams r = distribution ratio (0.68 male, 0.55 female average) β = elimination rate, ~0.015 g/dL per hour t = hours since the first drink
Alcohol distributes through body water rather than total body mass, so the formula divides the alcohol consumed by an estimate of body water — weight multiplied by a distribution ratio. Because women carry a lower average proportion of body water, the same drinks produce a higher concentration. The liver then removes alcohol at a roughly constant rate regardless of how much is present, which is why the decline is linear rather than exponential and why nothing except time speeds it up.
Exemple détaillé
A 180 lb (81.6 kg) man drinking 3 standard drinks over 2 hours: 42 g of alcohol divided by 55,500 g of body water gives a peak of 0.0757%, less 0.015 × 2 hours of metabolism — an estimated BAC of about 0.046%, taking roughly three more hours to reach zero.
BAC Calculator : le guide complet
Why you should not use this to decide whether to drive
The Widmark formula was derived from population averages, and individual results scatter around it substantially. Food in the stomach can delay absorption by hours, meaning your actual peak arrives later than calculated. Medications, liver condition, hydration, drinking pace, and body composition all shift the result in ways no simple formula captures.
More fundamentally, impairment does not begin at a legal limit. Measurable degradation of reaction time, divided attention, and risk judgement appears from around 0.02%, well below every legal threshold in the world. The limit is a prosecutable line, not a safety threshold.
The only reliable rule is the simple one: if you have been drinking, do not drive. Plan the ride home before the first drink, not after.
What counts as one standard drink
A US standard drink contains 14 grams of pure alcohol. That is a 12 oz beer at 5% ABV, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 40% spirits. All three deliver the same alcohol despite looking very different.
Real servings routinely exceed this. Craft beers commonly run 7–9% ABV, so a 16 oz pour can be two and a half standard drinks. A generous restaurant wine pour is often 8 oz rather than 5. Home-poured spirits are notoriously heavy. Counting glasses rather than standard drinks is the single most common way people badly underestimate their intake.
Standard-drink definitions also differ by country — the UK unit is 8 g, Australia's is 10 g. This calculator uses the US 14 g definition.
Only time removes alcohol
The liver metabolises alcohol at roughly 0.015 g/dL per hour — about one standard drink per hour for an average adult — and this rate is essentially fixed. It does not speed up because you want it to.
Coffee, cold showers, exercise, energy drinks, and eating afterwards do nothing to the elimination rate. Caffeine in particular is dangerous precisely because it masks sedation without reducing impairment, producing someone who feels alert and is still badly impaired.
The practical arithmetic is sobering: reaching 0.15% means roughly ten hours until zero. Someone drinking heavily until 2 am can still be over the legal limit driving to work the next morning, which is a large share of morning drink-driving arrests.
Legal limits around the world
The United States uses 0.08% for drivers 21 and over, with zero-tolerance laws below that age and a lower 0.04% limit for commercial drivers. Utah moved to 0.05% in 2018.
Much of Europe uses 0.05%, and several countries — including the Czech Republic, Hungary, and Romania — are effectively zero. Many jurisdictions apply lower limits to novice and professional drivers regardless of the general threshold.
Penalties commonly escalate sharply above 0.15%, and in most places refusing a breath test carries an automatic licence suspension of its own.
Questions fréquentes
How accurate is a BAC calculator?
Not accurate enough to rely on. The Widmark formula uses population averages and can be off by 30% or more in either direction for an individual. It cannot account for food, medication, drinking pace, or liver function. Never use a calculated figure to decide whether to drive.
How long does alcohol stay in your system?
Blood alcohol falls at roughly 0.015% per hour, so a BAC of 0.08% takes about five and a half hours to reach zero. Note that breath and blood tests measure current BAC, while urine tests can detect metabolites for up to 80 hours and hair tests for months.
Can I sober up faster with coffee or a cold shower?
No. Nothing accelerates the liver's elimination rate. Caffeine is actively risky because it makes an impaired person feel alert while leaving their reaction time and judgement just as degraded.
Why do women reach a higher BAC than men from the same drinks?
Alcohol distributes through body water, and women carry a lower average proportion of body water than men of the same weight. The Widmark ratio reflects this — 0.55 versus 0.68 — so the same alcohol produces a roughly 20% higher concentration. Women also have lower average levels of gastric alcohol dehydrogenase, which increases how much alcohol reaches the bloodstream.