How it works
Blood Alcohol Concentration (BAC) estimates the grams of alcohol per 100 millilitres of blood. The Widmark formula distributes consumed alcohol across total body water, which differs by sex: r ≈ 0.68 for men and 0.55 for women. This engine uses: BAC% = (drinks × 14 g) ÷ (r × weightKg × 1000) × 100 − 0.015 × hours, clamped at zero. The 14 g per drink is the US standard drink definition (roughly 12 oz beer at 5%, 5 oz wine at 12%, or 1.5 oz spirits at 40%). The −0.015 × hours term models average alcohol elimination (~0.015% BAC per hour, though individual metabolism varies 0.01–0.02). Worked example: 80 kg male, 4 drinks, 2 hours elapsed. Peak before elimination: (4×14)/(0.68×80000)×100 ≈ 0.103%. After 2 hours: 0.103 − 0.030 = 0.073%. Most US states set the illegal driving limit at 0.08% for adults 21+; Utah uses 0.05%. Zero tolerance applies under 21 in all states. This calculator is educational — it cannot account for food, medication, tolerance, carbonation, or testing device error. Never drive after drinking; individual BAC varies and impairment begins well below 0.08%.
Formula
BAC% = (drinks × 14) / (r × weightKg × 1000) × 100 − 0.015 × hours | r = 0.68 (M) or 0.55 (F)
Tips
- Food slows absorption — the same drinks on an empty stomach peak higher and faster.
- One drink per hour is a folk rule, not physics; weight and sex dominate the estimate.
- Impairment starts below 0.08% — legal limit is not a safe-driving threshold.
- Use rideshare or a designated driver; no calculator replaces a breathalyser or blood test.
Frequently asked questions
- What is my BAC after 4 drinks?
- For an 80 kg male over 2 hours: approximately 0.073% — just under the 0.08% US driving limit, though impairment may still be present.
- How fast does BAC drop?
- Average elimination is about 0.015% per hour. At 0.073%, roughly five more hours could reach near zero — highly individual.
- Why is female BAC higher for the same drinks?
- Women typically have less body water per kilogram (lower r), so alcohol concentrates more. The formula uses r = 0.55 vs 0.68 for men.
- Is this calculator legal evidence?
- No. It is an estimate for education. Law enforcement uses calibrated breath or blood tests; many factors this model ignores can shift real BAC significantly.