Blood Alcohol Content (BAC) measures the concentration of alcohol in your bloodstream. It determines intoxication level and legal driving limits. Understanding BAC helps you make safer decisions about alcohol consumption.
The Widmark Formula
BAC = (Alcohol consumed in grams) ÷ (Body weight in grams × r) − (β × hours)
Where:
- r = Widmark factor: 0.68 for men, 0.55 for women (distribution factor)
- β = Elimination rate: approximately 0.015% per hour (range: 0.010–0.030%)
Worked Example
A 75 kg man drinks 3 standard drinks (each containing 10 g alcohol) over 2 hours:
Alcohol = 3 × 10 = 30 g
BAC peak = 30 ÷ (75,000 × 0.68) = 30 ÷ 51,000 = 0.000588 = 0.059%
BAC after 2 hrs = 0.059% − (0.015 × 2) = 0.059% − 0.030% = 0.029%
At 0.029%, this man is well under the UK drink-drive limit of 0.08% (80 mg/100ml).
Standard Drink Reference
| Drink | Volume | ABV | Alcohol (grams) |
|---|---|---|---|
| Regular beer | 330 ml | 5% | 13 g |
| Wine (small) | 125 ml | 12% | 12 g |
| Wine (large) | 250 ml | 12% | 24 g |
| Spirit (single) | 25 ml | 40% | 8 g |
| Pint of beer | 568 ml | 4% | 18 g |
BAC and Its Effects
| BAC % | Effects |
|---|---|
| 0.02–0.03 | Slight relaxation, mild mood elevation |
| 0.05–0.07 | Relaxed, mild impairment to judgement |
| 0.08–0.10 | Impaired coordination, reaction time |
| 0.10–0.15 | Slurred speech, significant impairment |
| 0.15–0.20 | Nausea, loss of balance |
| 0.20+ | Stupor, unconsciousness possible |
| 0.30+ | Life-threatening |
Legal Limits by Country
| Country | Drink-drive limit (BAC%) |
|---|---|
| UK (England/Wales) | 0.08% |
| Scotland | 0.05% |
| USA | 0.08% |
| Germany | 0.05% |
| Australia | 0.05% |
| Sweden | 0.02% |
| Japan | 0.03% |
How Long to Sober Up
Your liver eliminates alcohol at roughly one standard drink per hour. There is no method to speed this up — coffee, water, and food do not change the rate.
Hours to sober = BAC ÷ 0.015
Example: BAC of 0.12% → 0.12 ÷ 0.015 = 8 hours to reach 0%
Important Disclaimer
BAC calculators provide estimates only. Individual variation is significant. Never use a calculation to decide whether it is safe to drive — err on the side of caution.