Blood Alcohol Content (BAC)

The formula

BAC% = (drinks × 14 g ÷ (weight in g × r)) × 100 − 0.015 × hours

Each drink is treated as 14 g of alcohol, and r is the Widmark factor: 0.68 for men and 0.55 for women, reflecting average body water. For the defaults: 42 g ÷ (70,000 × 0.68) = 0.000882, which is 0.0882%, minus 0.015 × 2 = 0.03, giving 0.058%.

Worked examples

3 drinks, 70 kg, 2 h, male → 0.058%
4 drinks, 70 kg, 2 h, male → 0.080%
3 drinks, 55 kg, 2 h, female → 0.079%
2 drinks, 90 kg, 1 h, male → 0.015%
6 drinks, 70 kg, 3 h, male → 0.132%

Why the estimate cannot clear you to drive

The formula assumes an average body composition, an average rate of elimination, and that every drink contains exactly 14 g of alcohol. None of those hold exactly for a real person. Body water varies with muscle mass and hydration, absorption depends on food and on how fast the drink was consumed, and elimination rates differ between individuals by a wide margin. Some people reach 0.08% on fewer drinks than the formula suggests; others on more. The result is a teaching estimate of the arithmetic, not a legal or medical assessment.

What the model leaves out

• Drink strength. A pint of strong beer, a large glass of wine and a cocktail are rarely one standard drink.
• Food and timing. A meal slows absorption and flattens the peak, so two people with identical drinks can differ noticeably.
• Elimination is not linear. The 0.015% per hour term is an average; real clearance varies and does not speed up with coffee, water or fresh air.
• Tolerance is not sobriety. Feeling unaffected changes the impairment very little.
• Legal limits differ. Many jurisdictions use 0.05% or lower, and commercial drivers often face stricter thresholds.

The only safe plan is not to drive after drinking at all. If you need a number for a legal or medical purpose, it has to come from a calibrated measurement, not this page.

Frequently Asked Questions