Blood Alcohol Content (BAC) Calculator
Roughly estimate blood alcohol content from standard drinks, bodyweight, sex and time using the Widmark formula.
How it works
BAC% = alcohol(g) ÷ (bodyweight(g) × r) × 100 − 0.015 × hours; alcohol(g) = drinks × 14; r = 0.68 (male), 0.55 (female)
The Widmark equation is the classic back-of-envelope estimate of blood alcohol. It counts about 14 g of alcohol per standard drink, spreads it through the water in your body — captured by r, which averages 0.68 for men and 0.55 for women — and then subtracts roughly 0.015 percentage points per hour as your liver clears it. It is an average model that ignores food, medications, drink strength and how fast you drank, so a real breathalyser reading can land well above or below it.
Worked example
A 75 kg man who has had 3 standard drinks (42 g of alcohol) in the past hour: 42 ÷ (75,000 × 0.68) × 100 ≈ 0.082, minus 0.015 for one hour of metabolism, leaving an estimated BAC of about 0.067%.
Frequently asked questions
Can I use this to decide whether to drive?
No. Never use any calculator to decide whether to drive. The estimate can be wrong in either direction, impairment often starts before the legal limit, and the only safe amount of alcohol before driving is none.
How long until my BAC is zero?
Alcohol clears at roughly 0.015 percentage points per hour and only time does it — coffee, food, water and cold showers do not speed it up. A BAC of 0.06% takes about four hours to fully clear.
Why do men and women get different numbers?
The factor r reflects body-water content, which is on average lower in women, so the same drinks spread through less water and produce a higher BAC. Individual body composition shifts this further.
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Educational estimate, not medical advice. Consult a qualified professional for health decisions.