The Widmark Formula: How BAC Is Actually Calculated

Blood Alcohol Concentration (BAC) is the most legally consequential number most people never learn to calculate. The legal limit for driving in most US states is 0.08%. Cross that line and you're facing DUI charges, license suspension, and thousands in fines. But what does 0.08% actually mean? How many drinks gets you there? And why does it vary so dramatically from person to person? The math behind BAC estimation was worked out by a Swedish physician named Erik Widmark in the 1930s. His formula is still the basis for every BAC calculator, every forensic alcohol analysis, and every expert witness testimony in DUI cases. It's surprisingly simple, and understanding it might be the most practically useful math you'll ever learn. The Widmark formula BAC = (A / (r * W)) - (beta * t) Where: A = total alcohol consumed in grams r = Widmark factor (body water constant): 0.68 for men, 0.55 for women W = body weight in grams beta = alcohol elimination rate (typically 0.015% per hour) t = time ela