Estimate blood alcohol content with the Widmark formula
This calculator estimates your blood alcohol content (BAC) from the number and strength of your drinks, your body weight and sex, and the time elapsed since your first drink. It is an educational tool for understanding how alcohol is processed, not a fitness-to-drive test.
How it works
First the tool finds the grams of pure alcohol you consumed:
grams = volume(ml) × ABV/100 × 0.789 × number of drinks
Then it applies the Widmark formula, where r is the body-water constant
(0.68 for men, 0.55 for women):
BAC% = (grams ÷ (weight in grams × r)) × 100 − 0.015 × hours
The final term subtracts an average elimination rate of about 0.015% per hour. Hours until sober is the peak BAC divided by 0.015.
Example
A 75 kg man drinks 3 × 330 ml beers at 5% ABV, 2 hours ago:
- grams = 330 × 0.05 × 0.789 × 3 ≈ 39.1 g
- peak BAC = 39.1 ÷ (75,000 × 0.68) × 100 ≈ 0.0766%
- after 2 hours: 0.0766 − 0.015 × 2 ≈ 0.0466% (below 0.05%)
- hours to sober ≈ 0.0766 ÷ 0.015 ≈ 5.1 hours
Important: this is an estimate only. Real BAC varies widely between people and situations. Never use it to decide whether you are fit to drive — if you have been drinking, do not drive. Everything runs in your browser; nothing is uploaded.
What affects real BAC beyond the formula
The Widmark formula is a population average; individual results can differ meaningfully because of factors the calculator cannot know:
Food in the stomach slows alcohol absorption significantly. Drinking on an empty stomach can produce a peak BAC 50% higher than the same drinks consumed with a meal, because food delays gastric emptying and the rate at which alcohol reaches the small intestine.
Body composition matters within the sex categories. The Widmark constant (0.68 for men, 0.55 for women) reflects the average water fraction of body weight. A leaner person with more muscle has a higher water fraction, so their BAC will be slightly lower than the formula predicts; a person with a higher fat proportion will trend slightly higher.
Medications and health conditions alter metabolism. Certain antihistamines, antibiotics, and antidepressants interact with alcohol. Liver conditions reduce metabolic rate. None of these are modelable with a simple formula.
Drinking rate matters because alcohol absorbs fastest in the first 30–60 minutes after consumption. The tool assumes absorption is complete — it applies metabolism from the elapsed time you enter, not from a dynamic absorption curve.
Understanding BAC bands
Different BAC levels correspond to typical effects, though individual tolerance varies widely:
| Approximate BAC | Typical effects |
|---|---|
| Under 0.02% | Little to no effect for most people |
| 0.02–0.05% | Mild relaxation, slight impairment of judgment |
| 0.05–0.08% | Reduced coordination, exceeds legal limit in many countries |
| 0.08–0.15% | Clearly impaired coordination, slurred speech, slower reactions |
| Above 0.15% | Significant impairment; risk of loss of consciousness at higher levels |
The UK legal limit for driving is 80 mg of alcohol per 100 ml of blood (0.08%), among the highest in Europe. Scotland, Ireland, and most of Europe use 50 mg/100 ml (0.05%). The only genuinely safe BAC for driving is zero.
Hours-to-sober caveat
The “hours until sober” estimate divides peak BAC by 0.015% per hour, which is an average elimination rate. Real rates range roughly from 0.010% to 0.020% per hour depending on the individual. Black coffee, cold showers, and food do not accelerate alcohol elimination from the bloodstream — only time does.