Estimating blood alcohol content
Blood alcohol content (BAC) is the share of alcohol in your blood, expressed in percent: 0.08% means 0.08 g of alcohol per 100 ml of blood. It depends on how much alcohol you drank, your body weight, your sex and how much time has passed. This calculator uses the Widmark formula, the classic method used in forensic science, to give an estimate.
This is an estimate only. Never use it to decide whether you can drive.
The Widmark formula
- Alcohol consumed (g) = number of drinks × grams per drink. A US standard drink contains 14 g.
- Peak BAC (%) = alcohol (g) ÷ (body weight in g × r) × 100, where r is the share of body water: about 0.68 for men and 0.55 for women.
- Current BAC = peak − 0.015 × hours since the first drink.
- Time to zero = current BAC ÷ 0.015 per hour.
Worked example
A man weighing 180 lb has 4 standard drinks over 3 hours.
- Alcohol: 4 × 14 = 56 g.
- Peak: 56 ÷ (81,647 × 0.68) × 100 = 0.101%.
- After 3 hours: 0.101 − 0.045 = 0.056%.
- Time until about zero: 3 h 43 min more.
He is under 0.08% but above Utah’s 0.05% limit, and his judgment and reaction time are impaired.
BAC by number of drinks (180 lb man, 3 hours)
| Drinks | Estimated BAC | Time to zero |
|---|---|---|
| 2 | 0.005% | 0:22 |
| 3 | 0.031% | 2:03 |
| 4 | 0.056% | 3:43 |
| 5 | 0.081% | 5:24 |
| 6 | 0.106% | 7:05 |
What is a standard drink?
In the US, 14 g of pure alcohol: 12 oz of regular beer at 5%, 5 oz of wine at 12%, or 1.5 oz of 80-proof spirits. Many drinks are larger: a 16 oz pint of IPA at 6.5 to 7% is about 1.8 standard drinks, and a restaurant pour of wine is often 6 to 9 oz.
Legal limits
The limit for drivers 21 and over is 0.08% in every state except Utah, where it is 0.05%. Commercial drivers are limited to 0.04%. Drivers under 21 face zero-tolerance laws, with limits of 0.00 to 0.02%. You can be arrested for impaired driving below these limits.
Why your real BAC can differ
- Food slows absorption and lowers the peak.
- Body composition: more body fat means less water to dilute alcohol.
- Medication, fatigue and health change how alcohol affects you.
- Elimination varies between about 0.010% and 0.020% per hour. Coffee, water and cold showers do not speed it up.
Stay safe
Plan your ride home before you start drinking. Use a designated driver, a ride-share or public transportation. If someone shows signs of alcohol poisoning, such as confusion, vomiting, slow breathing or unresponsiveness, call 911.