The Widmark formula is the standard educational approximation for estimating blood alcohol concentration (BAC) from drinks consumed, body weight, and time — the same underlying math taught in alcohol-safety education. Our BAC calculator applies it instantly, with clear limitations spelled out.
The formula
BAC% = (Alcohol grams ÷ (Body weight grams × distribution ratio r)) × 100 − (0.015 × hours elapsed)- Alcohol grams: standard drinks × 14g (a U.S. standard drink contains about 14 grams of pure alcohol)
- Distribution ratio (r): roughly 0.68 for males, 0.55 for females — reflecting average body water percentage differences, since alcohol distributes through body water
- Elimination term: the body metabolizes alcohol at roughly 0.015% BAC per hour, which is subtracted from the raw concentration
Example
A 180 lb male who has had 3 standard drinks over 2 hours: roughly 0.046% BAC — below the common 0.08% legal driving limit in this specific scenario, but still representing real impairment.
What counts as a standard drink
A U.S. standard drink contains about 14 grams of pure alcohol — roughly a 12 oz regular beer (5% ABV), a 5 oz glass of wine (12% ABV), or a 1.5 oz shot of 80-proof spirits (40% ABV). Stronger pours, larger glasses, or higher-ABV craft beers all contain more alcohol than one standard drink, which this calculator can’t detect on its own — enter your actual standard-drink-equivalent count.
Why this is only a rough estimate
Real BAC is affected by food intake (slows absorption), hydration, medications, individual metabolism, and the pace of drinking — none of which the simplified Widmark formula accounts for. It’s a widely used educational approximation, not a precise or medically/legally reliable measurement.
A critical safety note
Never use this calculator — or any online BAC estimate — to decide whether it’s safe to drive. If you’ve been drinking, don’t drive; use a rideshare, a designated driver, or public transit instead. This tool is for education only.