BAC Calculator
Blood alcohol concentration (BAC) depends on how much alcohol you drink, your body weight, your biological sex, and how much time has passed, this calculator applies the clinically established Widmark formula to give you a rough numerical estimate for educational purposes.
Formula
Worked example
3 drinks (42 g alcohol), 70 kg man (r = 0.68), some food (a = 0.93), 1 hour: 42 ÷ (70,000 × 0.68) × 100 × 0.93 − 0.015 ≈ 0.067%.
How the Widmark Formula Works
The calculator uses the Widmark formula, the same pharmacokinetic model underlying most forensic and clinical BAC estimation. Each standard drink contains approximately 14 grams of pure ethanol. That mass is divided by your body weight in grams and a sex-based distribution factor, roughly 0.68 for males and 0.55 for females, to reflect the fact that ethanol distributes into body water, and women typically carry a higher proportion of body fat relative to lean mass. A fixed liver-elimination rate of approximately 0.015% BAC per hour is then subtracted for each hour since drinking began.
How to Use This Calculator
Enter the number of standard drinks consumed, your body weight, your biological sex, and the number of hours that have passed since you started drinking. The result is expressed as a BAC percentage, for example, 0.08% is the legal driving limit in most U.S. states. Use consistent units: one standard U.S. drink equals 12 oz of regular beer (5% ABV), 5 oz of wine (12% ABV), or 1.5 oz of 80-proof spirits. Mixed drinks with multiple shots or high-ABV craft beers may count as more than one standard drink.
Factors That Affect Your Real BAC
Food in your stomach slows gastric emptying and meaningfully reduces the peak BAC you reach from a given number of drinks. Medications, including aspirin, acetaminophen, certain antihistamines, and many others, can impair alcohol metabolism or amplify its effects. Individual variation in the alcohol dehydrogenase enzyme, liver disease, and hormonal fluctuations across the menstrual cycle all cause real-world BAC to diverge from any formula's prediction. Fatigue, hydration status, and caffeine do not lower BAC, though they can mask the subjective sense of impairment.
Limitations and Safety Notice
The Widmark formula produces a statistical average, not a precise reading for any individual. Your actual BAC at a given moment may be meaningfully higher or lower than the estimate shown. This calculator is a general-information tool only and must never be used to determine whether it is safe or legal to drive, operate machinery, or perform any safety-sensitive task. If you need an accurate BAC measurement, only a calibrated breathalyzer administered by trained personnel or a blood test ordered by a clinician can provide one. If you or someone you know is concerned about alcohol use, the SAMHSA National Helpline (1-800-662-4357) provides free, confidential support 24 hours a day.
Frequently asked questions
What is a standard drink, and why does it matter for BAC?
In the United States, one standard drink is defined as any beverage containing 14 grams (0.6 fluid ounces) of pure ethanol. Keeping track in standard drinks, rather than glasses, cans, or pours, is important because vessel sizes and alcohol percentages vary widely, and the formula requires a consistent unit of alcohol mass to produce a meaningful result.
Why does biological sex affect the BAC estimate?
The Widmark distribution factor (r) differs by sex because ethanol distributes into total body water, and women on average have proportionally less body water than men of the same weight due to differences in body composition. This means the same amount of alcohol becomes more concentrated in the bloodstream in a female body, independent of weight. The calculator uses r ≈ 0.68 for males and r ≈ 0.55 for females, consistent with the published pharmacokinetic literature.
Can I use this calculator to decide whether I am safe to drive?
No. This calculator provides a rough educational estimate based on population averages and cannot account for the many individual factors, enzyme activity, medications, food, fatigue, and more, that influence actual impairment. Driving impairment can occur at BAC levels well below the legal limit of 0.08% in the United States, and some individuals are significantly impaired at levels this calculator might underestimate. Never rely on any formula or app to make a driving decision.