BAC Calculator
This BAC calculator estimates blood alcohol content with the Widmark formula from sex, body weight, drink volume and time. Example: an 80 kg man is at roughly 0.045% (0.43‰) two hours after starting on 1,000 ml of 5% beer. It is an estimate and never a basis for deciding to drive.
Example: Sex: male · Weight unit: kilograms (kg) · Body weight in kg 80 kg · Amount drunk 1,000 ml · Alcohol by volume 5 % ABV · Time since the first drink 2 hours → Estimate: BAC ≈ 0.045% (0.43‰). Source: Wikipedia – Blutalkoholkonzentration: Widmark formula with distribution factor r (men 0.68–0.7; women 0.55–0.60), elimination 0.1 to 0.2 ‰ per hour, density of whole blood 1.057 g/ml; original work: Widmark EMP. Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung. Berlin: Urban & Schwarzenberg; 1932. Updated: .
How it is calculated
A BAC calculator estimates blood alcohol concentration with the Widmark formula: grams of alcohol divided by body weight times a distribution factor, minus what the body has eliminated over time. The Swedish chemist Erik Widmark published the method in 1932, and it is still used for rough calculations today. Only a breath or blood test can actually measure the value.
Formula
Alcohol A = drink in ml × ABV ÷ 100 × 0.789 g/ml (the density of ethanol). Blood alcohol c = A ÷ (body weight in kg × r) − 0.15‰ × hours. Widmark’s distribution factor r is 0.68 for men and 0.55 for women. Elimination averages 0.15‰ per hour and ranges from about 0.10 to 0.20‰ between individuals. The result is in per mille (grams of alcohol per kilogram of blood); the percentage used in the United States (grams per 100 ml) is obtained by multiplying by 0.1057, so 1‰ is about 0.106%.
Worked example
Man, 80 kg, 1,000 ml of 5% beer, two hours since the first drink: A = 1,000 × 0.05 × 0.789 = 39.45 g. Without elimination that would be 39.45 ÷ (80 × 0.68) = 0.73‰. After two hours: 0.73 − 0.30 = 0.43‰, or about 0.045% BAC. Depending on the elimination rate the value lies between 0.33 and 0.53‰, and on paper it reaches zero after about 4.8 hours. For comparison: a 60 kg woman is at about 1.00‰ one hour after starting on 400 ml of 12% wine.
- The formula assumes all the alcohol is in the blood at once. In reality absorption takes up to two hours, so the peak is somewhat lower and later.
- Body fat, age, medication and illness change both distribution and elimination.
- Legal driving limits differ from country to country, and even low levels impair reaction time and judgement.
Frequently asked questions
How accurate is a BAC calculator?
It only gives a ballpark figure. Being off by several hundredths of a percent is normal, because drinking pace, food and individual elimination are not in the formula. Only a measurement is reliable.
How long does it take for alcohol to leave your system?
At the average rate of 0.15‰ per hour (about 0.016% BAC), a level of 1‰ takes just under seven hours to clear; depending on the person it is between roughly five and ten hours.
Can you speed up alcohol elimination?
No. The liver breaks alcohol down at a nearly constant rate. Coffee, water, fresh air or sleep do not change it.
What is the difference between per mille and BAC percent?
Per mille here means grams of alcohol per kilogram of blood, while the BAC figure common in the US is grams per 100 millilitres. 1‰ equals about 0.106% BAC.
Sources and legal basis
- Wikipedia – Blutalkoholkonzentration: Widmark formula with distribution factor r (men 0.68–0.7; women 0.55–0.60), elimination 0.1 to 0.2 ‰ per hour, density of whole blood 1.057 g/ml; original work: Widmark EMP. Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung. Berlin: Urban & Schwarzenberg; 1932
- Wikipedia – Blood alcohol content, section “Estimation” (Widmark model, mean elimination about 0.15 g/L per hour; more recent volumes of distribution 0.71 / 0.58 L/kg)
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