How the BAC Calculator Applies the Widmark Formula
Blood Alcohol Content, or BAC, is the concentration of ethanol in your blood expressed as a percentage by volume. It is the quantity breathalysers estimate and the quantity written into drink-driving law: 0.03% in India, 0.05% across most of Europe and Australia, 0.08% in the United States and England. This tool applies the Widmark formula, published by Swedish chemist Erik Widmark in the 1930s and still the basis of forensic alcohol calculations, to estimate BAC from your drinks, their size and strength, your body weight, your sex, and the hours elapsed. It exists for education. It is here so you can see how the variables interact, understand why the same round affects two people at the same table differently, and grasp how slowly alcohol actually clears. It is not a breathalyser, and the section below explains bluntly why it cannot be treated as one.
The estimate runs in three steps. First, drinks become grams of pure ethanol: grams = number of drinks × volume in ml per drink × (ABV% ÷ 100) × 0.789, where 0.789 g/ml is the density of ethanol at room temperature. Second, Widmark converts that mass into a blood concentration: BAC% = (grams of alcohol ÷ (body weight in grams × r)) × 100, where r is the Widmark factor, the proportion of your body that is water and therefore able to hold alcohol. It averages 0.68 for men and 0.55 for women, a difference driven by body composition, since fat holds very little water. Third, it subtracts what your liver has already cleared, at roughly 0.015% BAC per hour from when you started drinking. Alcohol elimination is close to zero-order kinetics, meaning it clears at a near-constant rate regardless of how much you drank, which is exactly why nothing speeds it up. The result is floored at zero rather than showing a negative value.
Work a case through. An 80 kg man drinks three 330 ml beers at 5% ABV over two hours. Ethanol mass = 3 × 330 × 0.05 × 0.789 = 39.06 g. Body water = 80,000 g × 0.68 = 54,400 g. So the raw concentration is (39.06 ÷ 54,400) × 100 = 0.0718%. Subtract two hours of elimination at 0.015%, which is 0.030%, and the estimate is 0.042%. Already above India's 0.03% limit. Now a 60 kg woman drinks two 150 ml glasses of wine at 12% ABV in one hour. Ethanol = 2 × 150 × 0.12 × 0.789 = 28.40 g, body water = 60,000 × 0.55 = 33,000 g, so raw BAC is 0.086%, less one hour of elimination gives 0.071%. She drank less total alcohol than he did and lands at nearly twice his estimated BAC, purely through weight and the r factor.
Where this is genuinely worth looking at. A group planning a dinner sees that two glasses of wine in an hour puts a 60 kg person near 0.07%, and books a cab before anyone orders. Someone who stopped drinking at 1 am works out that at 0.015% per hour, a BAC of 0.09% takes six hours to clear, which means driving to work at 7 am is still a real risk, and the myth of sleeping it off collapses. A student comparing three beers against two double whiskies discovers the spirits contain slightly less ethanol than the beer, which reframes what counts as a heavy night. A health educator uses the same formula to show a class why body weight and sex change outcomes at an identical table.
Now the part that matters most. Never use this, or any calculator, to decide whether you can drive. The model is a straight line through a complex biological process and it omits nearly everything that moves real BAC. Food in your stomach can slow absorption enough to halve a peak reading, and drinking on an empty stomach can push it far above the estimate. The 0.015% per hour elimination rate varies between roughly 0.010% and 0.020% between individuals, and enzyme activity differs with genetics, liver health, and regular drinking. Medications, particularly sedatives, antihistamines, and some antibiotics, interact with alcohol in ways no formula captures. Drinking speed, carbonation, hydration, illness, and where you are in the absorption curve all shift the number, and peak BAC arrives 30 to 90 minutes after your last drink, so an estimate taken too early understates what is coming. The r factors are population averages that assume average body composition. In short, the honest error band here is wide enough that a result of 0.04% could be 0.02% or 0.08% in reality. Impairment of reaction time and judgement also begins below every legal limit. If you have been drinking, do not drive: take a taxi, a rideshare, public transport, or a sober driver. Nothing you can do speeds up elimination, not coffee, not a cold shower, not food, not exercise, only time. This tool provides no medical or legal advice and must never be used to judge fitness to drive or operate machinery. Everything runs in your browser, so nothing you enter is uploaded or stored.
