How to use the BAC Calculator
- Count standard drinks, not glasses — a standard drink is 14 g of pure alcohol, roughly 350 ml of 5% beer, 150 ml of 12% wine or 45 ml of 40% spirits.
- Enter body weight in kilograms and select the biological sex used for the body-water constant.
- Enter the hours since your first drink, not since your last.
- Read the estimated current BAC, the peak before metabolism, and the projected time to sobriety.
- Treat every output as an estimate that cannot authorise driving — see the caveats below.
How the calculation works
The Widmark equation divides the grams of alcohol consumed by body mass multiplied by a distribution ratio r, which represents the fraction of the body that is water. Alcohol distributes into body water only, so r is about 0.68 for men and 0.55 for women — the difference is body composition, not tolerance. Elimination is then subtracted at a near-constant rate, conventionally 0.015 percentage points per hour, because the enzyme that clears alcohol saturates at low concentrations and works at a flat rate rather than proportionally.
That zero-order elimination is why 'sobering up fast' does not work. Coffee, cold showers, food after the fact and exercise change alertness but not clearance. Only time reduces BAC, and the rate is fixed within a fairly narrow band of roughly 0.012 to 0.020 per hour across individuals.
The estimate carries real error. Food in the stomach slows absorption and lowers the peak, drinking speed shifts the curve, medication and liver condition alter clearance, and pour sizes at home routinely exceed a standard drink by half. Widmark's own validation work showed individual results scattered around the prediction, so this calculator is an educational model and never a legal or safety instrument. If you have been drinking, do not drive.
BAC% = (grams of alcohol ÷ (body mass g × r)) × 100 − 0.015 × hours; r = 0.68 male, 0.55 femaleSource: Widmark EMP, Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung (1932); NIAAA definition of a US standard drink (14 g ethanol).
Worked example
A 75 kg man, three standard drinks, two hours after starting.
- Alcohol consumed = 3 × 14 = 42 g.
- Peak BAC = 42 ÷ (75,000 × 0.68) × 100 = 0.082%.
- Metabolised over two hours = 2 × 0.015 = 0.030.
- Current BAC = 0.082 − 0.030 = 0.052%.
About 0.052% — above the 0.05% limit in much of Europe and Australia, and roughly 3.5 hours from zero.
Frequently asked questions
Can I use this to decide whether to drive?+
No. It is an educational estimate with wide individual error, and impairment begins below every legal limit. If you have been drinking, arrange another way home.
Why is the constant different for men and women?+
Alcohol distributes into body water. Women average a lower proportion of body water for the same weight, so the same dose reaches a higher concentration.
Does food lower my BAC?+
Food eaten before or while drinking slows gastric emptying and lowers the peak. Food afterwards does not speed clearance.
How long does one drink take to clear?+
Roughly one hour per standard drink for an average adult, because elimination runs near 0.015% per hour.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.