Blood Alcohol Calculator

Blood alcohol concentration expresses grams of ethanol per 100 milliliters of blood, reported in the United States as a percentage, so 0.08 percent means 80 milligrams per deciliter. The Widmark model treats the body as a single compartment in which alcohol distributes only into body water. Dividing the dose in grams by the product of body mass and a distribution ratio r gives the peak concentration; r averages about 0.68 for men and 0.55 for women, reflecting that women carry a lower proportion of body water. Elimination is then subtracted at a near-constant 0.015 percent per hour, because the enzyme alcohol dehydrogenase saturates at low concentrations and the reaction proceeds at zero order rather than as a percentage decay. A US standard drink contains 14 grams of pure ethanol, equivalent to 12 ounces of 5 percent beer, 5 ounces of 12 percent wine, or 1.5 ounces of 40 percent spirits; a UK unit is 8 grams. The model has real limits. It assumes instantaneous absorption, but food in the stomach can lower and delay the peak substantially, and absorption may continue for 30 to 90 minutes after the last drink. Fixed r values ignore body composition, and measured elimination rates vary roughly from 0.010 to 0.025 percent per hour between individuals. Tolerance changes how impaired a person feels but not the concentration in the blood, which is what the law measures.

The formula is named for Erik Matteo Prochet Widmark, a Swedish physician and biochemist at Lund University, who developed micro-methods for measuring blood ethanol during the 1920s and published the definitive account of the distribution factor and linear elimination in a 1932 monograph on the medico-legal determination of alcohol. Sweden used that work to enact one of the earliest per se drink-driving limits in 1941, prosecuting on a measured concentration rather than on observed behavior. Enforcement needed a roadside test. Rolla Harger of the Indiana University School of Medicine developed the Drunkometer in 1931, and his colleague Robert Borkenstein produced the far more practical Breathalyzer in 1954, which fixed a blood-to-breath ratio and made mass testing feasible. Borkenstein's Grand Rapids study, published in 1964, quantified crash risk against BAC and supplied the evidence base for statutory limits. In the United States the threshold fell from 0.15 to 0.10 and then to 0.08 percent, which every state had adopted by 2004 under the pressure of federal highway funding conditions and campaigning by groups such as Mothers Against Drunk Driving. The National Transportation Safety Board recommended 0.05 percent in 2013, and Utah became the first state to adopt it, effective at the end of 2018.