BAC Calculation Guide and Resources


click here for more info on Campus FactoidsThe estimation of a person's blood alcohol content (BAC) has become particularly important in recent years for legal reasons. Increasingly strict drunk driving laws and stiffer legal consequences for those who commit crimes while alcohol-impaired have recently made the estimation of BAC very important. There are several methods that can be used for BAC calculation, although Stowell and Stowell (1998)* showed that none of the methods were of great accuracy and that none of the methods were superior to the others. There are many websites which contain a BAC estimator and there are many different opinions of how to accurately calculate BAC. The following are links to sites that contain useful information on how to estimate BAC.

click here for supplementary infoThe most commonly used formula is referred to as the "Widmark formula". It can be summarized in the following formula:#
[(# of drinks)(# oz. ethanol/drink)(0.0514 lbs. ethanol/oz.ethanol)(1.055 g ethanol/ml)]/[(weight of person in lbs.) * Widmark "r" value] * 100 - (B * time) where "r" refers to the percentage of body mass that contains alcohol (total body mass - mass of fat and bone), and B is the hourly decrease in BAC.

The US Department of Transportation uses a slightly different formula which is essentially an updated "Widmark formula": %
[(# of drinks)(# oz. ethanol/drink)(23.36 grams ethanol/oz.)(0.806 ml H2O/ml blood)]/[(weight of person in lbs./2.2046 lb/kg)(TBW)(1000g/kg)] * 100 - (B * time) where TBW refers to the total body water volume and B is the hourly decrease in BAC.

The two above methods are generally the most reliable and accurate, although complex factors such as alcohol clearance rate, total body water content, and food consumption will ultimately affect BAC at any given time and add to the inaccuracy of the estimation methods. An article by Professor Craig with additional information can be seen in the Alcohol Inquirer (ver 2).

*Ref: Stowell, A.R.; Stowell, L.I. Estimation of blood alcohol concentrations after social drinking. Journal of Forensic Sciences. 43 (1), pp. 14-21, 1998.
# Ref: Ed Kuwatch, Esq. Defense attorney (Widmark method)
%Ref: National Highway Traffic Safety Administration (Herbert Moskowitz, Ph.D.)

page last modified: 1/27/2000