Blood Alcohol Concentration: The Widmark Formula, Drink Counts, and Why Estimates Fail
October 29, 2026
Blood alcohol concentration (BAC) is the percentage of alcohol in the blood. A widely cited estimate is the Widmark formula: BAC (%) = [alcohol consumed in grams ÷ (body weight in grams × r)] × 100 − (0.015 × hours since drinking began), where r is the distribution ratio, about 0.68 for men and 0.55 for women, and 0.015 percentage points per hour is a typical elimination rate. It is a rough educational model; actual BAC varies substantially between people.
Important: this article explains the arithmetic and why it is unreliable. No calculation should be used to decide whether it is safe to drive. Impairment begins well below legal limits, and the only safe approach is not to drive after drinking.
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Open the Water Intake Calculator →A Worked Example
A standard drink in the United States contains about 14 grams of pure alcohol (a 12-ounce beer at 5%, a 5-ounce glass of wine at 12%, or 1.5 ounces of 80-proof spirits). A man weighing 81.65 kg (180 pounds), drinking four standard drinks (56 grams) quickly, would have a peak BAC of about 56 ÷ (81,650 × 0.68) × 100 = 0.101%, before accounting for elimination. After 3 hours, subtracting 0.015 × 3 = 0.045 gives 0.056%. A 60 kg woman (r = 0.55) drinking three drinks (42 grams) would reach 42 ÷ (60,000 × 0.55) × 100 = 0.127%.
The same drinks produce a higher BAC in a smaller person and in someone with a lower proportion of body water, which is on average the case for women.
| Person | Drinks (14 g each) | Peak BAC estimate (%) | After 3 hours (%) |
|---|---|---|---|
| 81.65 kg man | 2 | 0.050 | 0.005 |
| 81.65 kg man | 4 | 0.101 | 0.056 |
| 60 kg woman | 2 | 0.085 | 0.040 |
| 60 kg woman | 3 | 0.127 | 0.082 |
Why Estimates Are Unreliable
Actual BAC depends on factors the formula ignores: whether you have eaten (food slows absorption, lowering the peak), the speed of drinking, body composition, hydration, medications, genetics, liver function, and the exact alcohol content of the drink, since many craft beers and mixed drinks are stronger than the standard. Pour sizes at home are often larger than the standard. Elimination rates vary from about 0.010 to 0.025 percentage points per hour. Coffee, cold showers, and other remedies do not speed elimination. BAC can also keep rising for up to an hour or more after the last drink as alcohol is absorbed.
Impairment Starts Early
Even small amounts of alcohol impair reaction time, judgment, and coordination. In many countries the legal driving limit for most drivers is 0.08% (and lower in some, like 0.05% or 0.02%), and commercial drivers and drivers under the legal drinking age often face lower limits. Crash risk rises steeply with BAC: at 0.08%, the risk is several times that of a sober driver. Impairment depends on the person; being under the legal limit does not mean that you are safe to drive. If you plan to drink, arrange a ride, use a designated sober driver, or stay overnight.
Using the Knowledge Responsibly
The formula is useful for understanding how drink size, body weight, and time interact, for example how slowly the body clears alcohol (roughly one standard drink per hour). It should not be used to time driving or to justify drinking more. If you are concerned about your drinking, resources such as national helplines and healthcare providers can help.
Frequently Asked Questions
What is the Widmark formula?
An estimate of BAC from alcohol grams, body weight, a distribution ratio, and elapsed time at about 0.015% per hour elimination.
How fast does the body eliminate alcohol?
Roughly one standard drink per hour, or about 0.015 percentage points of BAC per hour.
Can coffee or a cold shower sober me up?
No. Only time lowers BAC.
Can I use a BAC calculator to decide if I can drive?
No. Estimates are unreliable, and impairment begins below the legal limit.
How much alcohol is in a standard drink?
About 14 grams in the United States.