What is Blood Alcohol Content (BAC)?
Blood Alcohol Content (BAC) is the metric used to measure the amount of alcohol present in a person's bloodstream. A BAC of 0.08% means that there are 0.08 grams of alcohol for every 100 milliliters of blood inside the body. In the United States (and many other countries), a BAC of 0.08% is the legal limit for driving a motor vehicle.
The Widmark Formula Explained
This calculator utilizes the widely accepted Widmark Formula, developed by Swedish physician Erik Widmark in the 1920s. While variations exist, the core mathematical equation remains the global standard for forensic BAC estimation.
The formula calculates total alcohol consumed in grams, divided by body weight in grams, multiplied by a gender-specific distribution constant (r), and then subtracts the metabolic elimination rate over time.
- Gender Constant (r): 0.68 for men, 0.55 for women. Women generally have a higher percentage of body fat and less body water than men of the exact same weight. Since alcohol is highly water-soluble, it becomes more concentrated in a female's blood, resulting in a higher BAC for the exact same amount of alcohol consumed.
- Elimination Rate: The human liver metabolizes alcohol at a relatively constant rate of approximately 0.015% BAC per hour, regardless of how much you have drank. Drinking coffee, taking a cold shower, or eating food will not speed up this elimination rate.
Understanding the Standard Drink
Not all drinks are created equal. The National Institute on Alcohol Abuse and Alcoholism (NIAAA) defines a "Standard Drink" as containing roughly 14 grams (0.6 fluid ounces) of pure alcohol. This is generally equivalent to:
- 12 fluid ounces of regular beer (at roughly 5% ABV)
- 5 fluid ounces of table wine (at roughly 12% ABV)
- 1.5 fluid ounces of 80-proof distilled spirits/liquor (at 40% ABV)
If you order a craft IPA that is 8% ABV, or a "double" mixed drink at a bar, you are actually consuming significantly more than one "Standard Drink," which will drastically alter your true BAC.