How to Compute BAC
Blood alcohol concentration, abbreviated BAC, is a measure of the grams of alcohol in the bloodstream per 100 milliliters. For example, a BAC of 0.03 means that a person has 0.03 grams of alcohol per 100 mL of blood. Some people mistakenly interpret this number as the percent of alcohol in the blood, that is, they assume that a BAC of 0.03 means 3% alcohol.
BAC is not usually measured directly, but instead estimated from your weight, the amount of alcohol consumed, and the time elapsed since you last drank. Your gender and whether you are a heavy or light drinker also affect your BAC. The BAC formula described below is based on the formula used by the National Highway Traffic Safety Administration. You can also estimate blood alcohol concentration with the convenient BAC calculator on the left.
Step 1Determine the total milliliters of water in your body from your weight (in pounds) and your gender. The formulas for total body water are
Body Water = 263.08627*Weight (males)
Body Water = 222.26254*Weight (females)
These expressions are based on averages and assume that the male body is about 58% water, and the female body about 49% water. Overweight people have less body water per pound, while leaner people have a higher percentage of water in their bodies.
Step 2Determine the number of grams of alcohol consumed. You can calculate this by multiplying the total ounces of alcohol by 23.36. For example, a 15 oz beverage that is 6% alcohol has 21.024 grams of alcohol since (15)(0.06)(23.36) = 21.024.
Step 3Compute the raw blood alcohol concentration (grams per 100 mL) with the formula
Raw BAC = 80.6(Grams of Alcohol)/(mL Body Water)
This formula assumes that blood is about 80.6% water.
Step 4Adjust the BAC to account for metabolism. How fast you metabolize alcohol depends on whether you drink heavily (more than 50 drinks per month), moderately, or very infrequently. If your last drink was H hours ago, then you must subtract a number that depends on H. For heavy drinkers, subtract 0.02H from the raw BAC. Moderate drinkers subtract 0.017H, and light drinkers subtract 0.012H.
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