Blood Alcohol Content (BAC) Calculator
Educational BAC calculator using the Widmark formula. Estimate blood alcohol content from number of drinks, body weight, biological sex, and hours elapsed. For educational use only — never drive after drinking.
Estimate your BAC using the Widmark formula based on drinks, weight, sex, and time.
Whether you’re planning a night out or just curious about how alcohol affects your body, this BAC calculator provides an educational estimate using the scientifically validated Widmark formula. Remember: this tool is for educational purposes only. Never use it to decide whether you are safe to drive.
What Is Blood Alcohol Content?
Blood Alcohol Content (BAC) is a measurement of the amount of alcohol present in your bloodstream, expressed as a percentage. A BAC of 0.08% means there are 0.08 grams of alcohol per 100 millilitres of blood.
BAC is the primary legal standard for impaired driving in most countries:
- USA, Canada, Australia: 0.08% legal limit
- UK: 0.08% in England, Wales, Northern Ireland; 0.05% in Scotland
- Most of Europe: 0.05%
- Many countries: 0.00% (zero tolerance)
Always check your local laws.
The Widmark Formula
This calculator uses the Widmark formula, developed by Swedish pharmacologist Erik Widmark in the 1920s:
BAC = (Drinks × 14) ÷ (Weight in kg × r × 1000) × 100 − (0.015 × Hours)
Where:
- 14 = grams of alcohol per standard drink
- r = body water constant: 0.68 for males, 0.55 for females
- 0.015 = average alcohol elimination rate per hour
What Counts as One Standard Drink?
A standard drink contains 14 grams of pure alcohol:
| Drink | Volume | Alcohol Content |
|---|---|---|
| Regular beer | 355 ml (12 fl oz) | 5% |
| Wine | 148 ml (5 fl oz) | 12% |
| Spirits | 44 ml (1.5 fl oz) | 40% |
Pint glasses, wine pours, and cocktails often contain more than one standard drink.
BAC Impairment Levels
| BAC Range | Category | Typical Effects |
|---|---|---|
| 0.00–0.01% | Sober | No measurable impairment |
| 0.01–0.03% | Minimal | Slight mood elevation |
| 0.03–0.08% | Impaired | Reduced coordination, slower reaction |
| 0.08–0.15% | Drunk | Significant impairment, slurred speech |
| 0.15–0.30% | Very drunk | Severe impairment, possible blackout |
| 0.30%+ | Dangerous | Risk of coma, respiratory failure |
How the Body Eliminates Alcohol
On average, the liver metabolises about 0.015% BAC per hour — roughly one standard drink per hour. This rate is consistent and cannot be accelerated by:
- Drinking coffee or water
- Cold showers
- Eating food after drinking
- Exercise
Only time reduces BAC. Food before drinking slows absorption but does not change elimination.
Example Calculations
Example 1: A 75 kg male drinks 3 standard drinks over 1 hour.
- BAC = (3 × 14) ÷ (75 × 0.68 × 1000) × 100 − (0.015 × 1)
- = 42 ÷ 51,000 × 100 − 0.015
- = 0.082 − 0.015 = 0.067% (impaired but legal in 0.08% jurisdictions)
Example 2: A 55 kg female drinks 4 standard drinks over 2 hours.
- BAC = (4 × 14) ÷ (55 × 0.55 × 1000) × 100 − (0.015 × 2)
- = 56 ÷ 30,250 × 100 − 0.030
- = 0.185 − 0.030 = 0.155% (very drunk, dangerous)
Frequently Asked Questions
Why do women typically reach higher BAC than men? Women generally have a lower body water ratio (r = 0.55 vs 0.68 for men) and often lower body weight. This means alcohol becomes more concentrated in a smaller water volume, resulting in higher BAC from the same number of drinks.
Why is the r value different for males and females? The Widmark r value represents the proportion of body weight that is water. Males have a higher muscle mass (which holds more water) and lower body fat percentage, resulting in a higher r value.
Can medications affect BAC? Yes. Many medications interact with alcohol, including antihistamines, antidepressants, pain relievers, and antibiotics. Even without affecting the Widmark calculation, medications can dramatically amplify alcohol’s impairing effects.
How long after drinking can I drive safely? There is no safe shortcut. Use the “hours until sober” output as a minimum guide, but always err on the side of caution. Individual variation in metabolism means the formula can be off by ±30%. If in doubt, do not drive.
Disclaimer
This calculator provides an educational estimate only. Actual BAC depends on many individual factors not captured by the Widmark formula, including body composition, food consumption, liver health, hydration, genetic variation, and individual tolerance. Never use this calculator to determine legal fitness to drive.