The Widmark Formula
Blood alcohol content is estimated from the alcohol consumed, body water, and the time elapsed:
BAC = (grams of alcohol / (body weight in g × r)) × 100 − (0.015 × hours)
The distribution ratio r reflects body water content: about 0.68 for men and 0.55 for women. This is the main reason the same drinks produce a higher BAC in a woman of the same weight — less body water to dilute the alcohol.
What a Standard Drink Is
A US standard drink contains 14 g of pure alcohol. All of the following are one standard drink:
| Drink | Volume | Strength |
|---|---|---|
| Beer | 355 ml (12 oz) | 5% |
| Malt liquor | 240 ml (8 oz) | 7% |
| Wine | 150 ml (5 oz) | 12% |
| Spirits | 44 ml (1.5 oz) | 40% |
Real servings routinely exceed these. A restaurant glass of wine is often 250 ml and a craft beer 6–8% — each closer to two standard drinks than one. Underestimating the count is the largest single source of error in any BAC estimate.
Elimination Is Fixed
The liver metabolises alcohol at roughly 0.015% BAC an hour — about one standard drink per hour. Nothing accelerates this. Coffee, cold showers, exercise, water and food all fail to change the rate; food only slows absorption if eaten before drinking, which lowers the peak rather than the total.
The practical consequence: someone at 0.12% needs about eight hours to reach zero, which is why morning-after driving remains a common offence.
Legal Limits
| Where | Limit |
|---|---|
| United States, most states | 0.08% |
| Utah | 0.05% |
| Most of Europe, Scotland | 0.05% |
| Sweden, Norway, Poland | 0.02% |
| Czechia, Hungary, Romania | 0.00% |
| Commercial drivers, US | 0.04% |
| Under 21, US | 0.00–0.02% |
This Is an Estimate, Not a Measurement
The Widmark formula assumes an average distribution ratio and a standard elimination rate. Real values vary substantially with body composition, liver function, medication, food, drink strength, carbonation and the pace of drinking. Errors of ±30% are entirely possible.
No calculator, app or personal breathalyser should be used to decide whether to drive. The only safe BAC for driving is zero.
Frequently Asked Questions
Does food lower my BAC?
Eating before drinking slows absorption and lowers the peak by up to a third. It does not reduce the total alcohol absorbed or speed elimination.
Why do women reach a higher BAC?
Lower body water percentage and less gastric alcohol dehydrogenase, so more alcohol reaches the bloodstream from the same drink.
Can I still be over the limit the next morning?
Easily. Ten drinks finishing at 2 a.m. can leave someone above 0.05% at 9 a.m. Elimination is slow and cannot be hurried.