The Science of BAC: Calculating Blood Alcohol Content
Estimate your current and future intoxication levels using our BAC Calculator, or read on to understand the complex biology and math behind alcohol metabolism.
Blood Alcohol Concentration (BAC) is the primary metric used by law enforcement and the medical community to determine intoxication. A BAC of 0.08%—the legal limit for driving in all 50 US states—means that there are 0.08 grams of alcohol for every 100 milliliters of your blood.
While a breathalyzer measures the physical alcohol vapor in your lungs, forensic toxicologists and mathematicians can estimate your BAC using a mathematical equation known as the Widmark Formula.
The Widmark Formula
Developed by Swedish pioneer Erik Widmark in the 1920s, the formula estimates BAC by taking into account the volume of alcohol consumed, the person's body weight, their biological sex, and the amount of time that has passed since drinking began.
The Base Formula:
BAC = [ (Alcohol consumed in grams) / (Body weight in grams × r) ] × 100
The Critical 'r' Variable (The Gender Distribution Factor)
The most important part of the Widmark formula is 'r', the gender distribution factor. This variable accounts for the biological difference in body composition between men and women.
Alcohol is water-soluble. Because the average male body contains more water (roughly 68%) than the average female body (roughly 55%, due to a naturally higher percentage of adipose fat tissue), alcohol becomes more diluted in a man's bloodstream.
- The standard 'r' constant for men is 0.68
- The standard 'r' constant for women is 0.55
This mathematical constant proves why a 150-pound woman will have a significantly higher BAC than a 150-pound man after drinking the exact same amount of alcohol in the exact same timeframe.
Factoring in Time (Metabolism)
The base Widmark formula tells you your peak BAC if you drank all the alcohol instantaneously. However, your liver is constantly metabolizing (burning off) alcohol.
To find your current BAC, you must subtract the alcohol your liver has already processed. The average human metabolizes alcohol at a highly consistent rate of roughly 0.015% BAC per hour.
The Final Formula:
Current BAC = (Peak BAC) - (0.015 × Hours passed since first drink)
What is a "Standard Drink"?
The entire calculation relies on knowing exactly how many grams of pure alcohol you consumed. In the United States, a "Standard Drink" contains 14 grams (or 0.6 fluid ounces) of pure alcohol.
However, different beverages have drastically different alcohol by volume (ABV) percentages. A Standard Drink is generally considered:
- 12 ounces of regular beer (approx. 5% ABV)
- 5 ounces of table wine (approx. 12% ABV)
- 1.5 ounces of 80-proof distilled spirits (40% ABV)
Pouring a 9-ounce glass of wine (a very common home pour) is actually mathematically closer to drinking two standard drinks, not one.
FAQ
Does eating food lower my BAC? Eating food before or during drinking does not change the Widmark formula or speed up your liver's metabolism. However, having food in your stomach slows the absorption of alcohol into the bloodstream, preventing a massive, rapid spike in peak BAC.
Disclaimer: Mathematical formulas cannot perfectly replicate human biology. Never rely on a calculator to determine if you are safe to drive. To explore hypothetical scenarios safely, use the CalcUnit BAC Calculator.