How BAC Is Calculated: The Widmark Formula Explained

Blood Alcohol Content (BAC) is the standard measure for how much alcohol is present in a person's bloodstream. Law enforcement and medical professionals worldwide rely on BAC to assess intoxication levels.

The Widmark Formula

The most widely used method for estimating BAC is the Widmark formula, developed by Swedish scientist Erik Widmark in the 1930s:

BAC = (A × 5.14) / (W × r) − 0.015 × H

Where:

• A = total alcohol consumed in ounces
• W = body weight in pounds
• r = gender-based distribution ratio (0.73 for men, 0.66 for women)
• H = hours since the first drink
• 0.015 = average alcohol elimination rate per hour

Key Factors That Affect BAC

Several variables influence how quickly your BAC rises and falls:

• Body weight: Heavier individuals generally reach lower peak BAC levels because alcohol distributes across more body mass.
• Biological sex: Women typically have a higher BAC than men after consuming the same amount, due to differences in body water content.
• Rate of consumption: Drinking faster raises BAC more sharply because the liver can only metabolize roughly one standard drink per hour.
• Food intake: Eating before or while drinking slows alcohol absorption, leading to a lower peak BAC.

What Counts as a Standard Drink?

In the United States, one standard drink contains approximately 0.6 ounces (14 grams) of pure alcohol. This is equivalent to:

• 12 oz of regular beer (5% ABV)
• 5 oz of wine (12% ABV)
• 1.5 oz of distilled spirits (40% ABV)

Limitations

The Widmark formula provides an estimate. Individual metabolism, medications, tolerance, and health conditions can cause actual BAC to differ significantly from calculated values. Never rely solely on a BAC calculator to decide whether it is safe to drive.