Widmark BAC Calculator

How the widmark bac calculator works

Erik Widmark's 1932 equation remains the standard first approximation in forensic toxicology. The mass of ethanol consumed is divided by the body's water-equivalent distribution volume, given by the factor r multiplied by body weight, then reduced by the amount already eliminated at rate β.

The factor r reflects that alcohol distributes into body water, not fat, which is why it differs between men and women and with body composition. β is typically 0.15 ‰ per hour but varies meaningfully between individuals and with drinking history. Both are the main sources of uncertainty in any back-calculation.

Frequently asked questions

What is the Widmark formula?

C = A ÷ (r × W) − β × t, where A is grams of ethanol consumed, r is the distribution factor, W is body weight in kilograms, β is the elimination rate, and t is hours elapsed.

What is the Widmark r factor?

It is the volume of distribution for alcohol as a fraction of body mass, around 0.68 for men and 0.55 for women. The difference reflects body water content rather than sex directly, so lean or heavier builds shift it.

How fast does the body eliminate alcohol?

Around 0.15 ‰ per hour, or roughly 0.015 % BAC per hour, in a typical adult. The range across individuals is wide, and chronic drinkers often clear faster.

What is the legal blood alcohol limit for driving in India?

0.03 % w/v, equal to 30 mg of alcohol per 100 ml of blood, under the Motor Vehicles Act.

Why does a Widmark estimate differ from a breath test?

Widmark assumes the full dose has been absorbed and that elimination is already linear. During absorption, in the first 30 to 90 minutes, the real level is lower than the calculated peak. Food, drinking pace, and individual metabolism all shift the curve, which is why only an evidential breath or blood analysis is admissible.

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