2025-01-15 · 6 min read
How BAC Is Calculated: The Widmark Formula Explained
Every online BAC calculator is built on the same foundation: the Widmark formula. Understanding how that formula works helps you interpret estimates more realistically and see where the model has limits.
The Core Formula
- A is total alcohol consumed in grams.
- W is body weight in grams.
- r is the Widmark distribution constant.
- T is hours elapsed since drinking began.
The Distribution Constant
The distribution constant estimates how alcohol disperses through body water. Because body composition differs on average, the standard Widmark model uses different constants for males and females.
Converting Drinks to Alcohol
A standard US drink contains about 14 grams of pure alcohol. BAC calculators first convert drink count and drink type into grams of alcohol before applying the formula.
The Metabolism Rate
The liver metabolizes alcohol over time, and a commonly used average is 0.015% BAC per hour. That is why only time lowers BAC in a meaningful way.
Limitations of Any Calculator
Food, medication, hydration, genetics, and drink size variation can all move actual BAC away from the estimate. Treat any calculator as a guide for awareness, not a precise measurement.