The Widmark formula
Blood alcohol content (BAC) is the percentage of alcohol in your bloodstream by volume. The most widely taught estimate is the Widmark formula, developed by Swedish scientist Erik Widmark in the 1930s. In its common US form it is:
BAC = ( A ÷ ( W × r ) ) × 100 − ( 0.015 × hours )
Where:
- A = grams of pure alcohol consumed. One US standard drink contains about 14 grams of alcohol.
- W = body weight in grams (pounds × 453.592).
- r = the Widmark factor, a body-water constant: about 0.68 for men and 0.55 for women, reflecting average differences in body composition.
- 0.015 × hours = alcohol eliminated over time. The body burns off roughly 0.015 BAC points per hour.
Worked example
A 160-pound man has 3 standard drinks over 1 hour:
Alcohol: 3 drinks × 14 g = 42 g.
Weight in grams: 160 × 453.592 ≈ 72,575 g.
Raw BAC: (42 ÷ (72,575 × 0.68)) × 100 ≈ 0.085%.
Less 1 hour of elimination: 0.085 − (0.015 × 1) = ≈ 0.070% BAC.
That sits just under the 0.08% legal driving threshold — but remember the estimate has a wide error band, and impairment begins well below 0.08%.
US legal limits & elimination
| Driver category | Per-se BAC limit |
|---|---|
| Drivers 21+ (most states) | 0.08% |
| Utah | 0.05% |
| Commercial drivers | 0.04% |
| Under 21 (zero-tolerance) | 0.00–0.02% |
Because alcohol leaves the body at a fixed rate, only time lowers BAC. Eating, coffee, water and cold showers may make you feel more alert but do not reduce the alcohol already in your blood.