Blood Alcohol Estimator

What the Widmark equation does and does not model

Widmark published it in 1932: BAC equals the alcohol consumed divided by body weight times a distribution factor, minus a constant elimination rate for each hour elapsed. The distribution factor r approximates the fraction of the body that is water — conventionally about 0.73 for men and 0.66 for women, reflecting average differences in body composition.

What it does not model: food in the stomach, which can delay and blunt absorption by a large margin; drinking pace, since it assumes all alcohol is already absorbed; medication interactions; liver function; genetic variation in alcohol dehydrogenase; and the individual variation in elimination rate, which spans roughly 0.010 to 0.020 per cent per hour. Real measured BAC commonly differs from a Widmark estimate by 20 per cent or more in either direction.

Time is the only thing that lowers BAC

Coffee, cold showers, food after the fact, exercise and fresh air do not change elimination rate. The liver processes alcohol at a roughly fixed pace regardless of what else you do. Coffee makes a person feel more alert while remaining equally impaired, which is more dangerous than feeling tired, not less.

Impairment starts below the legal limit

Measurable effects on divided attention, tracking and reaction time appear from around 0.02 per cent. At 0.05 per cent, coordination and response to emergencies are already reduced. The 0.08 per cent limit used in most US states is a legal threshold for prosecution, not a line below which driving is safe — and several jurisdictions set lower limits.

Why this page will not tell you it is fine to drive

An estimate with a 20 per cent error band cannot support that decision, and no calculator anywhere can. If you have been drinking, arrange another way home. That is the entire advice.

Frequently Asked Questions