You can estimate your blood alcohol concentration (BAC) using the Widmark formula, which combines the amount of alcohol you’ve consumed, your body weight, and a biological constant that accounts for how your body distributes alcohol. The legal limit for driving is 0.08% in every U.S. state except Utah, where it’s 0.05%. Here’s how the math works and what affects the result.
The Widmark Formula
The standard equation for estimating BAC is:
BAC = (A ÷ (W × r)) − (0.015 × H)
- A = total grams of pure alcohol consumed
- W = your body weight in grams (pounds × 453.6)
- r = the Widmark factor (0.68 for men, 0.55 for women on average)
- H = hours since your first drink
The 0.015 at the end represents your body’s average elimination rate per hour, meaning your BAC drops by roughly 0.015 percentage points for every hour that passes. Some people metabolize slightly faster or slower, with the range falling between 0.010 and 0.020 per hour.
How to Count Grams of Alcohol
One standard drink in the United States contains about 14 grams of pure alcohol. That’s the amount in a 12-ounce beer at 5% alcohol, a 5-ounce glass of wine at 12%, or a 1.5-ounce shot of spirits at 40% (80-proof). If you’re drinking something stronger or a larger pour, you need to adjust.
To calculate the grams of alcohol in any drink, multiply the volume in ounces by the alcohol percentage (as a decimal), then multiply by 23.36 (the weight of one ounce of pure ethanol in grams). A 16-ounce craft beer at 7% alcohol, for example, contains about 26 grams of alcohol, nearly two standard drinks.
A Worked Example
Say a 180-pound man has three standard beers over two hours. Here’s the calculation step by step:
Total alcohol: 3 drinks × 14 grams = 42 grams. Body weight in grams: 180 × 453.6 = 81,648. Widmark factor for men: 0.68.
BAC before elimination: 42 ÷ (81,648 × 0.68) = 42 ÷ 55,521 = 0.000756, or about 0.076%.
Subtract the metabolism over two hours: 0.076 − (0.015 × 2) = 0.076 − 0.030 = 0.046%.
His estimated BAC would be around 0.046%. Now run the same scenario for a 130-pound woman using the female Widmark factor of 0.55: 42 ÷ (58,968 × 0.55) = 42 ÷ 32,432 = 0.129%. After two hours of metabolism: 0.129 − 0.030 = 0.099%, which is over the legal limit.
Why the Widmark Factor Differs by Sex
The Widmark factor (r) reflects how much of your body is water versus fat. Alcohol dissolves in water but not in fat, so only the water-containing portion of your body dilutes the alcohol you drink. Men carry more lean mass and total body water on average, giving them a higher r value (0.68) and a larger “tank” to dilute alcohol into. Women typically have proportionally more body fat and less body water, resulting in a lower r value (0.55) and higher BAC from the same number of drinks.
This factor varies between individuals by as much as twofold, ranging from 0.4 to 0.8 depending on age, body composition, and overall size. When researchers dose alcohol based on total body water rather than just body weight, the age and sex differences in BAC nearly disappear. That tells you the real variable isn’t gender itself but how much water your body contains.
Body Composition and Age Matter
Two people who weigh exactly the same can reach very different BAC levels. A person with more muscle and less body fat has a larger volume of body water, so the same amount of alcohol gets diluted more and produces a lower BAC. Someone with a higher body fat percentage has less water to dilute the alcohol, pushing BAC higher.
This becomes especially relevant with aging. Lean body mass tends to decrease and fat tissue tends to increase as you get older, shrinking the volume of body water. The result is that an older adult will typically reach a higher BAC than a younger person of the same weight and sex drinking the same amount. Studies on elderly individuals have confirmed that the standard Widmark equation, which uses fixed r values, tends to underestimate BAC in older people for exactly this reason.
How Food and Timing Shift Your BAC
The Widmark formula assumes all the alcohol you drank has been absorbed into your bloodstream. In reality, absorption takes time. Alcohol can be measured in the blood within 30 to 70 minutes after a drink, but your peak BAC depends heavily on what’s in your stomach.
Drinking on an empty stomach means faster absorption and a higher, sharper peak. Eating before or while drinking slows absorption, flattening the curve so your BAC rises more gradually and peaks lower. This doesn’t reduce the total amount of alcohol your body processes, but it spreads the absorption over a longer window, giving your liver more time to keep up.
Drinking speed matters too. Spreading the same number of drinks over four hours versus two gives your body twice the metabolic runway, so your BAC at any given moment will be noticeably lower.
Why Manual Calculations Are Estimates
The Widmark formula gives you a reasonable ballpark, not a precise measurement. Several sources of error can push the result off:
- Pour size. Most people underestimate how much they’re actually drinking. A generous wine pour can easily be 7 or 8 ounces rather than the standard 5.
- Individual metabolism. The 0.015 per hour elimination rate is an average. Your actual rate could be 30% higher or lower.
- Body composition assumptions. Using the standard r values of 0.68 or 0.55 works for average-build adults, but these values can be significantly off for people who are very lean, very muscular, obese, or elderly.
- Absorption timing. The formula doesn’t account for alcohol still sitting in your stomach. If you just finished a drink minutes ago, your true BAC may still be climbing.
Because of these variables, your actual BAC could be meaningfully higher or lower than what the formula predicts. Breathalyzers and blood tests measure BAC directly rather than estimating it, which is why they remain the standard for legal purposes.
Legal BAC Limits in the U.S.
As of 2025, every U.S. state sets the legal BAC limit for adult drivers of noncommercial vehicles at 0.08%, with one exception: Utah uses a 0.05% limit. Commercial drivers face a stricter 0.04% threshold nationwide, and drivers under 21 are subject to zero-tolerance laws, where any detectable BAC can result in penalties.
Impairment starts well below the legal limit. Reaction time, coordination, and judgment begin to deteriorate at BAC levels as low as 0.02% to 0.03%, even if you feel fine. The legal threshold represents the point at which the law considers you presumptively impaired, not the point at which impairment actually begins.