Calculation of Blood Alcohol Concentration (BAC)

The Research by Widmark is 100 years old and his formula is listed below. I have found doing this calculation to be rewarding in that it gives you a sense of how much you can drink before you test above the allowable. You should note that doing this computation is only a guide and the results are not 100% accurate. Factors too numerous to mention and not brought into play. Experiment!

Widmark’s Basic Formula for calculating BAC (Blood Alcohol Concentration) at a particular time is expressed as follows:

%BAC = (A x 5.14/W x r) – .015 x H

“A” is the total number of liquid ounces of alcohol consumed by the person since the commencement of drinking. This figure is ascertained by multiplying the number of liquid ounces of alcoholic beverages consumed by the percentage of alcohol in the beverage. For example, a typical bottle of regular beer contains .60 liquid ounces of alcohol (12 oz. X 5%). A typical glass of table wine contains .60 liquid ounces of alcohol (5 oz. X 12%). And, most hard liquors contain 40 per cent alcohol by volume with a typical size of drink being 1.25 ounces and so a typical shot of hard liquor contains .50 ounces of alcohol (1.25 oz x 40%).

The 5.14 in the formula is a conversion factor of .823 x 100/16, wherein .823 is used to convert liquid ounces to ounces of weight, 100 is used to convert the final figure to a percentage, and 16 is used to convert pounds to ounces.

The 0.15 figure appearing in the formula is the average alcohol elimination rate.

“W” is for weight of the person in pounds

“r” in the alcohol distribution ratio which for men is .73 and for women is .66.

“H” is the period in hours during which alcohol was eliminated from the blodstream and this is usually the total number of hours between commencement of drinking and the time the BAC calculation is made.

Note: This is a very well-known formula. Widmark is very famous in this area of work. The formula is not full proof but only a guide.

Tutorial on Calculation Blood Alcohol Levels
A step by step tutorial explaining the formula to calculate blood alcohol level in the human body.

Guide to Calculating BAC While Out Drinking
Tips on how to calculate you blood alcohol level and know when it’s legally safe to drive.