Blood Alcohol Content Calculator Formula

Understand the math behind the blood alcohol content calculator. Each variable explained with a worked example.

Use the Blood Alcohol Content Calculator

Formulas Used

Estimated BAC

bac = max((alcohol_grams / (weight_grams * gender_r)) * 100 - 0.015 * hours_drinking, 0)

Hours Until BAC = 0

hours_to_sober = max((alcohol_grams / (weight_grams * gender_r)) * 100 / 0.015 - hours_drinking, 0)

Variables

VariableDescriptionDefault
drinksStandard Drinks(drinks)3
weight_lbsBody Weight(lbs)180
hours_drinkingHours Since First Drink(hours)2
gender_rGender0.73
weight_gramsDerived value= weight_lbs * 453.592calculated
alcohol_gramsDerived value= drinks * 14calculated

How It Works

Widmark Formula for BAC

BAC = (Alcohol in grams / (Body weight in grams x r)) x 100 - 0.015 x hours

Where:

  • 1 standard drink = 14 grams of alcohol
  • r = 0.73 for males, 0.66 for females (body water ratio)
  • 0.015 = average BAC reduction per hour

    • Legal Limits

    | BAC | Status | |---|---| | 0.00% | Sober | | 0.02-0.04% | Mild effects | | 0.05-0.07% | Impaired | | 0.08%+ | Legally intoxicated (most US states) |

    Worked Example

    A 180 lb male who had 3 drinks over 2 hours.

    drinks = 3weight_lbs = 180hours_drinking = 2gender_r = 0.73
    1. 01Alcohol = 3 x 14 = 42 grams
    2. 02Weight = 180 x 453.6 = 81,648 grams
    3. 03BAC = (42 / (81,648 x 0.73)) x 100 - 0.015 x 2
    4. 04= 0.0705 x 100 - 0.030 = 0.041%

    Ready to run the numbers?

    Open Blood Alcohol Content Calculator