Blood Alcohol Level (Widmark) Calculator
Estimate blood alcohol concentration (BAC) with the Widmark equation from standard drinks, beer, wine and spirits or grams of alcohol, body weight, sex and time — with an optional Watson body-water method, adjustable elimination rate, time until 0.00% and under a chosen limit, and an hour-by-hour BAC curve. An educational estimate only.
How it works
BAC% = grams of alcohol ÷ (weight in g × r) × 100 − β × hours since first drink · r = 0.68 men, 0.55 women (Widmark) or r = total body water ÷ (0.8 × weight) (Watson) · β ≈ 0.015% per hour · grams = mL × ABV% ÷ 100 × 0.789
The Widmark equation spreads the alcohol you drank through your body water and then subtracts what the liver has removed since the first drink. The distribution factor r is the share of body weight that alcohol can reach: on average 0.68 for men and 0.55 for women, who tend to carry more body fat. The Watson option replaces that average with your own total body water estimated from sex, age, height and weight, divided by the water content of blood (about 80%), which helps for people who are much leaner or heavier than average. You can enter drinks as standard drinks (with the country’s standard-drink size), as beers, glasses of wine and shots with their size and strength, or as grams of pure alcohol. The liver removes alcohol at a roughly constant rate of about 0.015% BAC per hour; you can change it under More options. The result shows your estimated BAC now, the theoretical peak, time to 0.00% and to below a limit you pick, the BAC in ‰, mg/100 mL and breath terms, and an hour-by-hour curve. It is an educational estimate: food, drinking pace, medicines and individual differences move real BAC up or down, and only a breath or blood test measures it.
Worked example
A 75 kg man has 3 US standard drinks (3 × 14 = 42 g of alcohol) and it has been 2 hours since the first one. Peak BAC = 42 ÷ (75,000 × 0.68) × 100 = 0.082%. The liver has removed 0.015 × 2 = 0.030%, leaving about 0.052% — already impaired and over a 0.05% limit — and it takes about 3.5 more hours to reach zero. With the Watson method (30 years, 175 cm), his body water is about 43.6 L, r = 43.6 ÷ (0.8 × 75) = 0.73, and the estimate is about 0.047% now.
Sources: Widmark equation — overview (NIH/PMC) · Searle J (2015) — Alcohol calculations and their uncertainty (Watson body water, Widmark factor) · Watson PE, Watson ID, Batt RD (1981) — Prediction of blood alcohol concentrations in human subjects · NIAAA — What is a standard drink?
Frequently asked questions
Can I use this to decide if I can drive?
No. This is an educational estimate with a wide error margin; individual BAC varies with food, genetics, medication and drinking speed. Impairment begins well below legal limits. If you have consumed any alcohol, do not drive — use a taxi, rideshare or designated driver.
Does coffee or a cold shower sober you up faster?
No. Only time lowers BAC, at roughly 0.015% per hour. Caffeine may make you feel more alert while still legally and functionally impaired, which is arguably more dangerous.
What counts as one standard drink?
In the US about 14 g of pure alcohol — roughly a 355 mL (12 oz) regular beer, a 150 mL (5 oz) glass of wine, or a 44 mL (1.5 oz) shot of spirits. Other countries use different sizes (10 g in Australia, 8 g for a UK unit), which you can choose. Cocktails, craft beers and large wine pours often contain more than one.
What is the difference between the Widmark and Watson methods?
Both use the same equation. Widmark uses an average distribution factor for each sex, while Watson estimates your total body water from age, height and weight. Watson tends to give a lower BAC for tall or lean people and a higher one for shorter or heavier people with more body fat.
Why is my BAC still rising after I stop drinking?
Alcohol is absorbed from the stomach and small intestine over 30–90 minutes, longer after food. The Widmark equation assumes instant absorption, so in the first hour or so the real BAC may still be climbing toward the estimated peak.
How do ‰ and breath readings compare with BAC %?
0.08% is 0.8 ‰ (0.8 g per litre of blood) and 80 mg per 100 mL. Breath tests in the US use 0.08 g per 210 L of breath, the same number, while the UK uses 35 µg per 100 mL of breath, which corresponds to the 80 mg/100 mL blood limit at a 2,300:1 ratio.
Related calculators
Educational estimate only — actual blood alcohol concentration varies widely between people and situations. Never use this to decide whether it is safe or legal to drive.