What is the Widmark formula?

Widmark's equation estimates blood alcohol concentration as the mass of alcohol consumed divided by the product of body weight and a distribution ratio r, then reduced over time by the body's elimination rate. It is the classic forensic model for BAC.

Why does sex change the result?

Alcohol distributes into body water, and the average proportion of body water differs by sex. The distribution ratio r is about 0.68 for men and 0.55 for women, so for the same dose and weight a woman's estimated BAC is higher.

What elimination rate does it use?

It assumes a steady 0.015 percent per hour, the commonly cited average. Real elimination ranges from about 0.010 to 0.025 percent per hour and depends on the individual, food intake, and chronic drinking patterns.

Can I rely on this to decide whether to drive?

No. This is an educational estimate with wide error bars and it ignores absorption time and many individual factors. The only safe rule is not to drive after drinking; use an evidential breathalyser if you must measure.

How many grams of alcohol are in my drink?

A UK unit is 8 g of pure ethanol; a US standard drink is 14 g. You can also compute it as volume in millilitres times alcohol percentage divided by 100 times 0.789, the density of ethanol.

Blood Alcohol Concentration Estimator (Widmark)

The Widmark formula is the foundational model used in forensic medicine and clinical toxicology to estimate blood alcohol concentration (BAC) from the amount consumed. This calculator turns a count of standard drinks, your body weight, sex, and elapsed time into a peak and a time-adjusted BAC estimate, all computed locally in your browser.

How it works

The formula first converts BAC at peak (ignoring elimination), then subtracts the alcohol the liver has cleared since drinking began:

A      = drinks × grams of alcohol per drink
peak   = (A / (r × weight_in_grams)) × 100
BAC(t) = peak − (0.015 × hours)

Here r is the Widmark distribution ratio — about 0.68 for men and 0.55 for women — which models the fraction of body mass that alcohol distributes into (total body water). The factor of 100 converts grams per gram into the conventional grams-per-100-mL percentage. The elimination term assumes 0.015% per hour.

Example, limits, and notes

A 75 kg man who drinks three UK units (8 g each, 24 g total) reaches a peak BAC of about 0.047%. After one hour of elimination that falls to roughly 0.032%. The model assumes alcohol is absorbed instantly, which overstates BAC in the first 30–60 minutes and ignores the food in your stomach, your drinking history, and individual metabolism. Treat the output as a teaching estimate with error bars of tens of percent — never as a legal or driving-fitness measurement.