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

    BACStatus 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%

    Frequently Asked Questions

    How long does it take to sober up?

    Your body eliminates alcohol at roughly 0.015% BAC per hour. There is no way to speed this up. Coffee, food, and cold showers do not lower BAC.

    What is the legal limit?

    In most US states, the legal BAC limit is 0.08% for adults. Some states have lower limits. For commercial drivers it is 0.04%. For under-21 it is typically 0.02% or zero.

    How accurate is this calculator?

    The Widmark formula provides an estimate. Actual BAC varies with food intake, drinking speed, medications, liver function, and individual metabolism.

    Learn More

    Guide

    How to Calculate Blood Alcohol Content (BAC)

    Learn how to estimate Blood Alcohol Content (BAC) using the Widmark formula. Understand how body weight, sex, drink count, and time affect BAC, and what legal limits mean in practice.

    Ready to run the numbers?

    Open Blood Alcohol Content Calculator