BAC Estimator
Widmark blood alcohol concentration estimate from drinks and weight.
Overview
A blood alcohol concentration (BAC) estimator turns a list of drinks into an approximate percentage of alcohol in the bloodstream, factoring in body weight, biological sex, and the time elapsed since drinking began. It is a useful tool for understanding how quickly the body metabolises alcohol, why a single drink hits people differently, and roughly when a person might return to a sober baseline after a night out.
The estimate is built on the Widmark equation, the same formula taught in toxicology courses since the 1930s. Because real BAC depends on dozens of factors the formula cannot capture — food in the stomach, hydration, medications, genetics — the result should always be treated as a rough planning figure rather than a substitute for a breathalyser, and never as permission to drive.
How it works
Widmark's formula is BAC% = (A / (W × r)) × 100 − (β × t), where A is grams of pure alcohol consumed, W is body weight in grams, r is the volume-of-distribution constant (about 0.68 for men and 0.55 for women), β is the elimination rate (typically 0.015% per hour), and t is hours since the first drink. Each standard drink contributes roughly 14 grams of ethanol, computed as volume_ml × ABV × 0.789 / 100.
Doses are summed and the elimination term is subtracted linearly, since the liver clears ethanol at a near-constant zero-order rate once enzymes are saturated. The result is then clamped at zero so the curve never goes negative.
Examples
- A 75 kg man drinks two 355 ml beers at 5% ABV over one hour. Pure alcohol is about 28 g; peak BAC is roughly 0.055% then drops by 0.015% per hour.
- A 60 kg woman has one 150 ml glass of wine at 12% ABV. Pure alcohol is about 14 g; estimated peak BAC is around 0.043%.
- A 90 kg man drinks four 44 ml shots of 40% spirits across two hours. Total alcohol is about 55 g; peak BAC reaches roughly 0.10% before clearance.
- The same 60 kg woman waits three hours after her glass of wine: 0.043% − (0.015 × 3) clamps to 0%.
FAQ
Is this safe for deciding whether to drive?
No. Treat the number as educational. Real BAC varies widely and legal limits differ by jurisdiction.
Why are male and female constants different?
Women generally have a higher proportion of body fat and lower total body water, so the same alcohol mass distributes into a smaller volume.
Does food change the result?
Eating slows absorption and lowers peak BAC but does not change how much alcohol the liver must clear.
Why is the elimination rate constant?
Alcohol metabolism is enzyme-limited (zero-order) at typical drinking concentrations, so removal is roughly linear over time.