BAC Calculator

BAC (%) = alcohol (g) ÷ (weight (kg) × r × 10) − 0.015 × hours

alcohol

Pure alcohol consumed, in grams (g)

weight

Body weight in kilograms (kg)

r

Widmark distribution factor: 0.68 for men, 0.55 for women

0.015

Alcohol cleared per hour, as a percentage

Use it to get a sense of what a given amount of drinking does to a reading, or how long a reading takes to fall. It describes an average body, so it cannot answer whether you are under a legal limit right now — only a breath test or a blood test can.

1. A 70 kg man, two 330 mL cans of 5 percent beer, one hour after the first drink

   1. Pure alcohol: 330 mL × 2 × 5% × 0.789 = 26.037 g
   2. Divide by weight and r: 26.037 ÷ (70 × 0.68 × 10) = 0.0547%
   3. Subtract one hour of clearance: 0.0547 − 0.015 = 0.0397%
   4. In the other unit: 0.0397% = 39.7 mg/100 mL
   5. Time to zero: 0.0397 ÷ 0.015 = 2.65 hours, about 2 h 39 min

   Two cans of beer leave a 70 kg man at 0.04 percent. That is under the 0.05 percent line used in some places, but over China's 20 mg/100 mL threshold — the same drinking, two different verdicts, because the limits differ.

2. The same drinks in a 70 kg woman

   1. The alcohol is identical: 26.037 g
   2. Divide by weight and the female r: 26.037 ÷ (70 × 0.55 × 10) = 0.0676%
   3. Subtract one hour of clearance: 0.0676 − 0.015 = 0.0526%
   4. In the other unit: 0.0526% = 52.6 mg/100 mL
   5. Time to zero: 0.0526 ÷ 0.015 = 3.51 hours, about 3 h 30 min

   The distribution factor is the only thing that changed, and it lifts the reading by a quarter. r is a population average, so an individual can sit well to either side of it.

3. A 60 kg woman, three 45 mL measures of 40 percent spirits, two hours in

   1. Pure alcohol: 45 mL × 3 × 40% × 0.789 = 42.606 g
   2. Divide by weight and r: 42.606 ÷ (60 × 0.55 × 10) = 0.1291%
   3. Subtract two hours of clearance: 0.1291 − 0.030 = 0.0991%
   4. In the other unit: 0.0991% = 99.1 mg/100 mL
   5. Time to zero: 0.0991 ÷ 0.015 = 6.6 hours — well into the next day

   Three measures of spirits take longer to clear than a working day is long, and sleeping does not shorten it. Only time does.

This is not a test and it cannot tell you whether you are fit to drive. Three things it cannot see decide the real number: absorption, since a reading keeps rising for up to an hour after the last drink while the formula assumes it all arrived at once; the distribution factor r, which varies by more than 30 percent between individuals; and how fast you actually clear alcohol, which food, liver health and drinking habits all change. Only a breath or blood test measures a real concentration.

Can I use this to decide whether to drive?

No — never. A BAC calculator describes an average body and cannot establish your own concentration, so it must not be the thing that decides whether you drive. Absorption, body composition and liver clearance all move the real figure, and a reading this page puts at 0.07 % can be 0.09 % in the person holding the phone. If you have been drinking, do not drive.

How long does it take to sober up?

Divide the reading by 0.015 per hour, the average rate at which the liver clears alcohol. At 0.08 % that is five and a half hours; at 0.15 % it is ten. Nothing speeds this up: coffee, a cold shower, food and sleep do not lower the concentration, they only change how you feel about it.

Why is the reading different for men and women?

The Widmark factor r stands for how much of the body is water, because alcohol dissolves in water rather than fat. Typical values are 0.68 for men and 0.55 for women, so the same drinks at the same weight give a higher concentration in a woman. Body composition moves r within each sex as well.

What counts as one standard drink?

It depends where you are. In the United States a standard drink holds 14 grams of pure alcohol, which is about a 355 mL can of 5 percent beer. China's dietary guidelines, in their 2022 edition, put the daily ceiling at 15 grams of alcohol for all adults regardless of sex — the earlier edition's 25 grams for men no longer applies — so a 500 mL bottle of 5 percent beer at roughly 20 grams is already past the whole day's allowance.

Does eating before drinking change the number?

It changes the curve, not the total. Food slows absorption, so the peak is lower and arrives later, while the formula here assumes everything is already absorbed at the start. A full stomach therefore makes the estimate read high in the first hour or two. It does not reduce how much alcohol you eventually have to clear.

• 车辆驾驶人员血液、呼气酒精含量阈值与检验（GB 19522-2024） — 国家市场监督管理总局、国家标准化管理委员会
• Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework — Jones AW — Forensic Science International, 2010
• Potential health impacts of changing the alcohol free descriptor: evidence review — Office for Health Improvement and Disparities, UK — Appendix with the Widmark equation and r values