Methodology

Alcohol mass equals volume × strength × 0.789 g/mL, the density of ethanol. BAC before elimination equals 100 × mass ÷ (Widmark factor × body weight), in mg per 100 mL. Elimination then removes 14.8 mg per 100 mL for each hour since the first drink. Sex and body mass index set the Widmark factor, following the relationship Forrest published and Searle (2015) restated: men 1.0181 − 0.01213 × BMI, women 0.9367 − 0.0124 × BMI. For the likely range, published uncertainties (strength ±3%, pour size ±5%, Widmark factor ±9.2%, elimination ±22%, with the r-β covariance) propagate through the calculation. Two standard deviations — about ±40%, matching Gullberg (2007) — define the band. Full absorption is assumed; shortly after drinking, the true value can still be rising.

Understanding Your Results

Read the estimate as a band, not a point: the true value can lie anywhere inside the range. Forensic literature treats roughly ±40% as the honest statement of what a Widmark estimate can know. Country comparison covers three limits where published: standard, professional, and novice drivers. When the whole range sits below a limit, the display says "below". When the range crosses it, the display says so — precisely the case where a single-number calculator would mislead. Two sober times appear on purpose. A typical time uses the average elimination rate; a cautious time assumes a slow eliminator at the edge of the range. When anything important depends on being sober, use the cautious time.

Practical Examples

Example 1: a woman of 65 kg and 168 cm has two 150 mL glasses of 12% wine. Two hours after the first glass, the estimate reads 0.37 g/L with a likely range of about 0.19-0.56 g/L. That range crosses the common 0.5 g/L limit, so "probably under" is all the numbers can honestly say. Zero comes typically about 4.5 hours after the first glass; cautiously, about 8. Example 2: a man of 82 kg and 180 cm has three 500 mL beers at 5% over three hours. His estimate reads 0.57 g/L (range 0.29-0.85), over the 0.5 g/L limit. Typical time to 0.0: about 7 hours; cautious: about 12. Example 3: a man of 75 kg and 180 cm has four 40 mL shots of 40% spirits over two hours, about 3.6 standard drinks. The estimate reads 0.64 g/L (range 0.41-0.87), clearly over the 0.5 g/L limit. Zero comes typically about 4.3 hours after the first drink; cautiously, about 9. Example 4: a woman of 60 kg and 165 cm has one 330 mL beer at 5%, one hour in. The estimate reads 0.18 g/L (range 0.09-0.27), under the common 0.5 g/L limit, yet over the 0.0 limit that Czechia, Hungary and Romania set, and that novice and professional drivers face in many countries. Even one drink is not "zero".

Tips for meaningful estimates

Read the strength from the label: beers range from about 4% to over 8%, wines from 9% to 15% — that difference changes the result substantially. Count what was actually poured. Home pours usually exceed standard servings; when unsure, round up. If you plan to drive later, decide the drinks from the plan, not the plan from the drinks. With a planned driving time, only little alcohol early in the evening — or none — keeps the cautious estimate at zero. Eating before and while drinking lowers the peak (NIAAA), but it does not make alcohol disappear faster.

Sources

• Searle (2015) — Alcohol calculations and their uncertainty, Medicine, Science and the Law
• Gullberg (2007) — Uncertainty of Widmark calculations, Forensic Science International
• Forrest (1986) — The estimation of Widmark’s factor, Journal of the Forensic Science Society
• European Transport Safety Council — BAC drink-driving limits across Europe
• NHTSA — Drunk driving: effects of BAC and crash risk
• CDC — Impaired driving: the US legal limit
• NIAAA — What is a standard drink?
• NIAAA — Understanding the dangers of alcohol overdose
• NIAAA — Food and peak alcohol level