A Widmark-formula estimate of blood alcohol concentration — useful for understanding how alcohol behaves, and useless for deciding whether to drive.
Erik Widmark published the underlying work in the 1930s, and the formula still bears his name. It rests on a simple idea: alcohol distributes itself through the body's water, so the concentration in blood depends on how much alcohol you drank divided by how much body water you have.
BAC % = alcohol (g) ÷ (body mass (g) × r) × 100 − 0.015 × hours
The r term is the distribution ratio — the fraction of body weight that is water. It averages about 0.68 in men and 0.55 in women, a difference driven mostly by body composition rather than sex as such: fat tissue holds little water, and women on average carry a higher proportion of it. The 0.015 is the elimination rate, in percentage points per hour, and it is roughly constant because the enzyme that does the work is saturated at almost any drinking level.
A 70 kg man drinks three US standard drinks — 42 g of alcohol — over two hours. Peak BAC is 42 ÷ (70,000 × 0.68) × 100 = 0.088%. Subtract two hours of elimination at 0.015 and the estimate now is 0.058%. Returning to zero from the peak takes about 5 hours 53 minutes from the first drink.
The same three drinks in a 60 kg woman peak at 0.127% — nearly 45% higher, from the combination of lower body mass and lower r. This is the single most important thing the formula shows, and it is why "keeping up" is a genuinely bad idea.
Alcohol is metabolised primarily by alcohol dehydrogenase in the liver, at a rate that is essentially fixed. Nothing meaningfully accelerates it. Coffee makes you a more alert drunk, not a soberer one — caffeine antagonises the sedation while leaving coordination and judgement exactly as impaired, which arguably makes things worse by removing the signal that you should stop. Cold showers, exercise, water and food after the fact do nothing for BAC.
Food eaten before drinking does help, but by a different mechanism: it slows gastric emptying so alcohol reaches the bloodstream more gradually, lowering the peak. It does not reduce the total amount absorbed. The only thing that removes alcohol is time.
| Limit (BAC) | Where |
|---|---|
| 0.00% | Saudi Arabia, UAE, Czechia, Hungary, Romania, Brazil (effectively) |
| 0.02% | Sweden, Norway, Poland, China, Estonia |
| 0.03% | India, Japan, Serbia |
| 0.05% | Most of Europe, Australia, South Africa, Thailand |
| 0.08% | United States, Canada, England and Wales, Mexico, New Zealand |
Scotland uses 0.05%, unlike the rest of the UK. Most jurisdictions also apply lower limits — often zero — to novice drivers, commercial drivers and anyone under a specified age. Limits change; check the current rule where you are rather than relying on a table.
Widmark's r is a population average applied to an individual, and body composition varies far more than the two values suggest. Beyond that: drink sizes poured at home bear little relation to standard measures; wine and spirits vary in strength; food, hydration and medication all shift absorption; liver disease slows elimination substantially; and some medications interact with alcohol in ways that alter both the metabolism and the effect.
Tolerance is a further trap. A regular drinker may feel and appear far less impaired than their BAC implies, but the legal limit is a blood concentration, not a behaviour — and reaction time is impaired regardless of how sober someone feels. Feeling fine is not evidence of being under the limit.
No, and it is worth being blunt about why. This is a population-average formula applied to an individual, and real BAC commonly differs from the estimate in either direction — meaning it can tell you that you are under a limit when you are over it. Drink measures poured at home are rarely standard, absorption varies with food and medication, and tolerance masks impairment without reducing it. The only reliable rule after drinking is not to drive.
Roughly one hour per standard drink, though it depends on body weight and sex. Elimination proceeds at about 0.015 percentage points of BAC per hour and is essentially fixed — the enzyme responsible is saturated at almost any drinking level, so the rate does not increase with how much you have had. A peak BAC of 0.12% therefore takes about eight hours to clear entirely, which is why people are sometimes still over the limit the following morning.
Not for BAC. Coffee counteracts drowsiness without touching coordination, reaction time or judgement, which can be worse than useless because it removes the tiredness that would otherwise stop you. Cold showers and exercise do nothing. Food eaten before drinking genuinely helps by slowing absorption and lowering the peak, but food eaten afterwards does not reduce alcohol already in the blood. Only time works.
Two reasons that compound. Body water is where alcohol distributes, and women on average carry a higher proportion of fat, which holds little water — that is what the lower r value of 0.55 against 0.68 represents. Lower average body mass adds to it. There is also evidence that women have less gastric alcohol dehydrogenase, so slightly more of what is drunk reaches the bloodstream rather than being broken down in the stomach.
It depends entirely on the country, which is why this calculator asks. A US standard drink is 14 g of pure alcohol — about 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of spirits. A UK unit is 8 g, and most of Europe and Australia use 10 g. Home pours are typically considerably larger than any of these, which is a major reason real BAC often exceeds what people expect.
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