Blood alcohol content depends on far more than just the number of drinks consumed — body weight, biological sex, and the time elapsed since drinking started all factor into the standard estimation formula, which is why the same number of drinks can affect two people very differently.
The Widmark formula, developed in the 1930s and still used as the basis for most BAC estimates today, calculates the amount of alcohol consumed relative to total body water, then adjusts downward for the rate at which the body metabolizes alcohol over time — roughly 0.015% BAC per hour for most people.
How BAC Is Estimated
BAC = [(Grams of Alcohol ÷ (Body Weight in grams × Distribution Ratio)) × 100] − (0.015 × Hours Since First Drink)
The distribution ratio (r) accounts for differences in body water percentage: approximately 0.68 for men and 0.55 for women, reflecting typical differences in body composition. A standard drink is defined as roughly 14 grams of pure alcohol — about a 12 oz beer, a 5 oz glass of wine, or a 1.5 oz shot of spirits.
A Worked Example
For a 170 lb (77.1 kg) man who has had 3 standard drinks (42g alcohol) over 2 hours: BAC ≈ [(42 ÷ (77,100 × 0.68)) × 100] − (0.015 × 2) ≈ 0.080% − 0.030% ≈ 0.050%. This is an estimate only — actual BAC varies significantly based on food intake, metabolism, hydration, and individual physiology, and should never be used as the basis for a decision about driving.
Common Mistakes to Avoid
- Using any BAC estimate to decide whether it's safe to drive: this tool is for education only — actual BAC varies by individual factors this formula can't fully capture, and impairment can begin well below the legal limit.
- Assuming food doesn't matter: eating before or while drinking slows alcohol absorption meaningfully, which this basic formula doesn't account for.
- Forgetting metabolism rate varies between individuals: the 0.015%/hour figure is an average — actual elimination rates vary based on liver function, genetics, and other factors.
- Not accounting for drink strength accurately: a "standard drink" assumes typical alcohol content — craft beers, fortified wines, and strong cocktails often contain meaningfully more alcohol than the standard reference amount.
Bottom Line
BAC estimation is genuinely useful for education about how alcohol and body weight interact — but it's an estimate, never a real-time safety measurement. Use a BAC Calculator to understand the factors involved, and always rely on a designated driver or rideshare rather than any BAC estimate when alcohol is involved.