Blood Alcohol Calculator

Estimating BAC after a few drinks involves the Widmark formula, body weight, gender (which affects body water composition), and time since drinking — variables most people don't track in real time. The result is people guessing whether they're under the legal limit by feel, which is famously unreliable.

Provide drinks (number, type, alcohol percentage), body weight, gender, and time elapsed, and the calculator returns estimated BAC plus an approximate time until your level returns to zero. Calculations use the standard Widmark formula with typical body water distribution coefficients (0.55 for males, 0.49 for females), so results are population averages, not personalized for your metabolism.

This is an educational estimate, not a legal defense or a green light to drive. Real BAC varies with food intake, fitness, medications, and individual differences in alcohol dehydrogenase activity. The only reliable measurement is a calibrated breathalyzer or blood test. If you're close to a legal limit, you're already too close.

The Widmark formula (developed in the 1930s by Swedish physician Erik Widmark) gives BAC = (alcohol mass in grams / (body weight × distribution coefficient)) − (elimination rate × hours since drinking). Distribution coefficient r reflects body water fraction — alcohol distributes through body water rather than total mass — and is 0.55 for typical men and 0.49 for typical women on average, reflecting average body composition differences. Elimination is roughly 0.015 BAC per hour for most adults, regardless of starting concentration; the body breaks down alcohol at a fixed enzymatic rate. The formula's accuracy depends on how well your body fits the population averages baked into r.

Worked example: a 180 lb (82 kg) man drinks three 12 oz beers at 5% ABV over an hour, then waits two more hours before driving. Alcohol per beer ≈ 14 g (a US standard drink), total 42 g. r = 0.55. Peak BAC ≈ 42 / (82000 × 0.55) × 1000 ≈ 0.093 (in BAC units, or 0.093%). Subtract elimination over the 3-hour window (assume drinking averaged across hour 1, then 2 hours of pure elimination): 3 × 0.015 = 0.045. Estimated BAC at end ≈ 0.048. That's below the 0.08 legal limit but within the impairment-detectable range. The takeaway: legal limits aren't safety limits; impairment starts much earlier.

The formula's limits are meaningful for individuals. Body water fraction varies with body composition (muscle has more water than fat), age (older people have less body water), hydration state, and sex. Two 180-lb men can have BAC differing by 30%+ at the same dose because of body composition differences. Food in the stomach slows alcohol absorption, lowering peak BAC by 30-50%. Medications and conditions affecting alcohol dehydrogenase change elimination rate. All of these are reasons to treat the calculation as a rough estimate, not a precise number, and certainly not a basis for "I can drive."

The legal context matters too. Most US jurisdictions enforce 0.08 as the per-se DUI limit, but commercial drivers face 0.04, drivers under 21 face zero-tolerance laws, and several jurisdictions are debating 0.05 limits aligned with European standards. Even at 0.05, impairment is real and measurable — reaction time, lane discipline, and decision-making degrade. The legal threshold is a policy compromise, not a safety threshold. If you've been drinking, the safe answer is don't drive; everything else is rationalization.