BAC Formula

Health Formula • Published on September 01, 2026 • Last updated September 01, 2026

Mathematical Equation

$$BAC\% = \frac{\text{Alcohol Consumed (g)}}{\text{Body Weight (g)} \times r} \times 100 - (0.015 \times \text{Hours})$$

Variable Definitions

Alcohol Consumed

Total grams of pure alcohol = Volume (mL) × ABV% × 0.789 (alcohol density), summed across all drinks

Widmark factor for body water distribution: 0.68 for men, 0.55 for women

Hours

Hours elapsed since drinking began, used to subtract metabolized alcohol at 0.015%/hour

Detailed Explanation

In-Depth Guide

The Widmark formula estimates Blood Alcohol Concentration by dividing the total grams of alcohol consumed by body weight adjusted for a gender-based body-water distribution constant, then subtracting the amount the body has already metabolized over elapsed time at an average elimination rate. It is widely used to give a rough estimate of impairment level, though it is not a substitute for an actual breathalyzer or blood test.

How to Calculate: Step-by-Step

1. For each drink, calculate grams of alcohol = Volume (mL) × (ABV% / 100) × 0.789. 2. Sum the alcohol grams across all drinks consumed. 3. Convert body weight to grams (kg × 1000, or lbs × 453.592). 4. Select the Widmark factor r: 0.68 for men, 0.55 for women. 5. Calculate Peak BAC% = (Total Alcohol g / (Weight g × r)) × 100. 6. Subtract metabolized alcohol: Current BAC = Peak BAC − (0.015 × Hours since first drink).

Worked Calculation Example

Male, Weight = 80 kg, 3 standard beers (330 mL each, 5% ABV), 2 hours elapsed: - Alcohol per beer = 330 × 0.05 × 0.789 ≈ 13.0 g - Total Alcohol = 13.0 × 3 = 39.1 g - Weight in grams = 80,000 g - Peak BAC = (39.1 / (80,000 × 0.68)) × 100 ≈ 0.072% - Current BAC = 0.072 − (0.015 × 2) = 0.072 − 0.030 = 0.042%

Common Use Cases

Getting a rough estimate of impairment level after drinking
Estimating how many hours until BAC returns to zero
Understanding how weight and gender affect alcohol processing

Frequently Asked Questions

Women generally have a higher proportion of body fat and lower body water content than men at the same weight, so alcohol becomes more concentrated in their bloodstream — the Widmark r-factor (0.55 vs 0.68) accounts for this difference.

No — this is only an estimate. Actual BAC is affected by many factors not captured here (food intake, medication, metabolism, drink strength variability), so it should never be used to decide whether it's safe or legal to drive.

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