Blood Alcohol Content (BAC) Calculator
The Blood Alcohol Content (BAC) calculator uses the Widmark formula to estimate the concentration of alcohol in your bloodstream based on your sex, body weight, number of standard drinks consumed, and the time elapsed since your first drink. A standard drink contains 14 grams of pure alcohol — equivalent to a 355 mL regular beer, a 148 mL glass of wine, or a 44 mL shot of spirits.
The body eliminates alcohol at an average rate of 0.015% BAC per hour. The Widmark factor (r) accounts for the different proportions of body water between males (r = 0.68) and females (r = 0.55), reflecting differences in body composition. This estimate is for educational purposes only; individual results vary significantly based on factors such as food intake, hydration, medication, and genetics.
How it works
BAC (%) = [drinks × 14 g] ÷ [weight_kg × 1000 × r] × 100 − 0.015 × hours. Where r = 0.68 (male) or 0.55 (female). Result is clamped to 0 if negative (alcohol fully metabolised).
Use cases
- Understanding how many drinks raise BAC above the legal driving limit
- Estimating how long to wait before driving after consuming alcohol
- Health education and awareness about alcohol metabolism
- Comparing the effect of body weight and sex on alcohol absorption
- Academic or research reference for the Widmark BAC formula
Frequently asked questions
How is blood alcohol content (BAC) calculated?
The Widmark formula estimates BAC as: grams of alcohol consumed ÷ (body weight in kg × 1000 × r) × 100, minus 0.015% for each hour since drinking began. The factor r reflects body water proportion — 0.68 for males and 0.55 for females. Each standard drink contributes 14 grams of pure alcohol to the calculation.
How long does it take for alcohol to leave your system?
The body eliminates alcohol at an average rate of about 0.015% BAC per hour, a rate that cannot be sped up by coffee, cold showers, or exercise. At that pace, a BAC of 0.08% takes roughly 5 to 6 hours to reach zero. Elimination speed varies somewhat between individuals depending on liver function and genetics.
What counts as one standard drink?
A standard drink contains 14 grams of pure alcohol. That equals a 355 mL regular beer (about 5% ABV), a 148 mL glass of wine (about 12% ABV), or a 44 mL shot of distilled spirits (about 40% ABV). Stronger craft beers or large pours can count as more than one standard drink.
How many drinks does it take to reach 0.08% BAC?
For a 70 kg (154 lb) man, each standard drink raises BAC by roughly 0.029% before elimination, so about three drinks in quick succession can approach the 0.08% limit used in many jurisdictions. A 60 kg woman gains roughly 0.042% per drink (r = 0.55), reaching similar levels with only two drinks. Food intake, drinking speed, and individual metabolism shift these numbers considerably.
Is a BAC calculator accurate enough to decide if I can drive?
No. The Widmark formula is an educational estimate, and real BAC varies significantly with food intake, hydration, medication, and genetics. Never use a calculator result to decide whether to drive — the only safe BAC for driving is zero, and only a certified breathalyzer or blood test measures actual levels. Consult a healthcare professional for questions about alcohol and your health.