Use the Widmark formula: BAC = (Alcohol consumed in grams ÷ (Body weight in grams × r)) × 100 − (metabolism rate × hours)
Convert drink alcohol to grams: grams of alcohol = volume × alcohol by volume (ABV) × 0.789
Use the body distribution factor r: about 0.68 for men and 0.55 for women
Use a typical metabolism rate of about 0.015 BAC per hour
Calculate total alcohol consumed from all drinks
Convert body weight to grams
Subtract the estimated amount metabolized over time
Adjust for time since drinking began
Use a BAC calculator or breathalyzer for a more accurate estimate