Blood Alcohol Content (BAC) Calculator
Use this blood alcohol content calculator to estimate your approximate BAC from drinks consumed, body weight, and time elapsed.
Updated October 2, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Blood Alcohol Content Is Calculated
Blood alcohol content (BAC) is the percentage of alcohol in a person's bloodstream. This calculator uses the Widmark formula, a long-standing estimation method that combines how much alcohol was consumed, body weight and sex, and how much time has passed since drinking began.
What This Calculator Measures
This gives a rough, educational estimate of BAC, not a precise or legally reliable reading. Actual BAC depends on many individual factors this formula cannot capture.
The Formula
BAC% = [alcohol consumed (g) / (body weight (g) x r)] x 100, minus (0.015 x hours elapsed)
r = 0.68 (men), 0.55 (women)
This is the Widmark formula, developed in the 1930s and still the basis for most BAC estimation tools¹. The constant r reflects how alcohol distributes through body water, which differs by sex on average.
How This Calculator Computes Your Result
Enter your sex, body weight, number of standard drinks, and hours since you started drinking. We convert drinks to grams of alcohol, apply the Widmark formula, and subtract the estimated amount already eliminated based on time elapsed. If the result would be negative, we show 0%.
Why This Is an Estimate, Not a Measurement
The classic Widmark r values (0.68 for men, 0.55 for women) are averages, not fixed biological constants. Later research measuring actual r values across real subjects found meaningful individual variation, with mean values sometimes running higher than the classic figures and a wide spread between individuals¹. Forensic toxicology literature treats Widmark-based BAC estimates as carrying substantial inherent uncertainty for any single person², which is why this tool is built for general awareness, not for any decision where precision matters.
A Note on Accuracy
This is a rough approximation for educational purposes only, and is explicitly not valid for legal, medical, or driving-safety decisions. Actual BAC varies substantially with food intake, drinking rate, medications, and individual body composition, factors this formula cannot capture. Never use a BAC estimate to decide whether it's safe to drive.
Frequently Asked Questions
Is this BAC calculator accurate enough to know if I can drive?
No. This is an educational estimate only, with a wide margin of error for any individual. Never use it to decide whether you are safe to drive, the only safe choice after drinking is not to drive.
How is BAC calculated?
Using the Widmark formula, which estimates BAC from grams of alcohol consumed, body weight, a sex-specific distribution constant, and time elapsed since drinking began¹.
Why does sex affect the BAC estimate?
The Widmark formula uses different distribution constants for men and women, reflecting average differences in body water content, though individual variation within each sex is substantial¹.
Does BAC decrease at a steady rate over time?
This calculator assumes a standard elimination rate of about 0.015% per hour, a commonly used average, though actual elimination rate varies by individual.
Sources
The Calculation of Blood Ethanol Concentrations in Males and Females, Seidl, Jensen & Alt, International Journal of Legal Medicine, 2000
Estimating the Uncertainty Associated with Widmark's Equation as Commonly Applied in Forensic Toxicology, Gullberg, Forensic Science International, 2007