How to Calculate Blood Alcohol
What is Blood Alcohol?
A Blood Alcohol Content (BAC) calculator estimates alcohol concentration in the blood using the Widmark formula. BAC is affected by drinks consumed, time elapsed, body weight, and sex.
Formula
BAC = (Alcohol grams) / (Body weight kg × r) × 100 − (0.015 × hours), where r=0.68 (male), 0.55 (female)
- BAC
- Blood Alcohol Content (%)
- A
- Alcohol consumed (grams)
- W
- Body weight (kg)
- r
- Distribution ratio — 0.68 male, 0.55 female
Step-by-Step Guide
- 1BAC = (Alcohol consumed in grams) / (Body weight in g × r) − (0.015 × hours)
- 2r = 0.68 for men, 0.55 for women (body water distribution)
- 3Standard UK unit = 8g alcohol; US standard drink = 14g
- 4Legal UK driving limit: 80mg/100mL blood (0.08%)
Worked Examples
Input
2 pints beer (4.5%), 80kg male, 1 hour
Result
Alcohol ≈ 45.4g; BAC ≈ 45.4/(80,000×0.68) − 0.015 ≈ 0.068%
Frequently Asked Questions
Why do women get drunk faster?
Women have lower body water % (r=0.55 vs 0.68 for men) and typically lower body weight, concentrating alcohol more.
What's the UK legal driving limit?
80 mg/100 mL blood (0.08% BAC) in England/Wales/Scotland. Scotland's limit is lower: 50 mg/100 mL.
Can food affect BAC calculation?
Food slows absorption but doesn't reduce total BAC. It delays peak BAC by 30-60 minutes.