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.

How standard drink definitions vary by country

The amount of pure ethanol in a “standard drink” differs significantly between countries, which directly affects any BAC calculation:

When entering data, confirm which standard applies to your drinks. A US standard drink contains nearly twice the alcohol of a UK unit, so confusing the two produces a substantial error in the estimate.

Understanding the elimination rate

The Widmark formula uses a fixed elimination rate of 0.015% per hour, sometimes written as β = 0.015 g/dL/h. This is a population average. In practice, the elimination rate varies considerably:

• Lower rates (≈0.010–0.012%/h) — less common; can occur with liver impairment or in people who drink very rarely.
• Average rates (≈0.015%/h) — the Widmark default; appropriate for most adults in a forensic estimate when no other data is available.
• Higher rates (≈0.017–0.025%/h) — more common in people who drink regularly, because chronic alcohol use up-regulates certain liver enzymes.

In forensic backtracking — calculating what someone’s BAC was at a time before a blood test — experts sometimes use a range of β values to produce a BAC range rather than a point estimate.

BAC legal limits by jurisdiction

This tool is for education only, but for context:

Legal limits are defined by evidential measurement (breathalyser or blood test), not Widmark estimates. Never rely on a formula calculation to assess fitness to drive.