Quick answer: Three US standard drinks (42 g of alcohol) for an 80 kg man after 2 hours: 42 ÷ (80,000 × 0.68) × 100 − 0.015 × 2 = 0.047% BAC, or 0.47 g/L.
How it's calculated
Widmark formula: BAC (%) = alcohol in grams ÷ (body weight in grams × r) × 100 − 0.015 × hours. The factor r is the share of body weight in which alcohol spreads: 0.68 for men and 0.55 for women on average. 0.015% per hour is a typical rate at which the body removes alcohol. Some calculators use other values of r (around 0.7 or more for men) or estimate it from height and weight, so their results can differ by 5–10%.
A standard drink holds a fixed amount of pure alcohol: 14 g in the US, 10 g in the WHO and Australian definitions, and 8 g in a UK unit. A large glass of wine or a strong beer can be two standard drinks or more.
BAC in % means grams per 100 mL of blood; multiply by 10 for grams per litre (per mille), the unit used in many countries. Legal limits for drivers differ by country, and some countries allow no alcohol at all.
Real values vary widely with food in the stomach, drinking speed, body composition, medicines and liver function; elimination rates range roughly from 0.010% to 0.020% per hour. The estimate cannot tell whether you are fit to drive or work.
Worked examples
- Man, 80 kg, 3 × 14 g, after 2 h: 42 ÷ (80,000 × 0.68) × 100 − 0.03 = 0.047%
- Woman, 60 kg, 2 × 14 g, after 1 h: 28 ÷ (60,000 × 0.55) × 100 − 0.015 = 0.07%
- Same woman, time to about zero: 0.085% ÷ 0.015 = 5.7 h from the first drink
Grams of alcohol in a drink
Grams of alcohol = volume (mL) × alcohol by volume (%) ÷ 100 × 0.789 (the density of ethanol). For example, 355 mL of 5% beer holds 355 × 0.05 × 0.789 = 14 g.
Frequently asked questions
Why is the result higher for women?
On average women have less body water per kilogram, so the same amount of alcohol is more concentrated in the blood. The Widmark factor r reflects this (0.55 versus 0.68).
Does coffee or water lower BAC faster?
No. The liver removes alcohol at a roughly steady rate; coffee, water or a cold shower do not change it.
What does a BAC of 0.08% mean?
0.08 g of alcohol per 100 mL of blood (0.8 g/L). It is a common legal limit for drivers in some countries, but impairment starts well below it.
How long does alcohol take to leave the body?
On average BAC falls by about 0.015% an hour, so a BAC of 0.08% takes a little over 5 hours to reach zero. Sleep, coffee and exercise don't speed this up much.
What counts as one standard drink?
It depends on the country: 14 g of alcohol in the US, 10 g in the WHO and Australian definitions, and 8 g for a UK unit. Choose the one that matches how you counted.
Can I use this result to decide whether to drive?
No. It is an average estimate, and real levels vary from person to person. Several countries, including in the Gulf, allow no alcohol at all when driving.