Blood alcohol level, or blood alcohol concentration (BAC), is estimated using a formula that accounts for how much you drank, your body weight, your biological sex, and how much time has passed. The most widely used method is the Widmark formula, which gives a reasonable estimate but comes with real limitations worth understanding.
The Widmark Formula
The standard equation for estimating BAC looks like this:
BAC = (Alcohol consumed in grams / (Body weight in grams × r)) × 100
Then, to account for time:
BAC = (Alcohol in grams / (Body weight in grams × r)) × 100 − (0.015 × hours since first drink)
The variable “r” is a distribution constant that reflects how much of your body is water (since alcohol dissolves in water, not fat). The standard values are 0.7 for biological males and 0.6 for biological females. Women generally have a higher proportion of body fat and less body water per pound, which means the same amount of alcohol concentrates more in their bloodstream.
How to Convert Drinks Into Grams of Alcohol
Before you can use the formula, you need to know how many grams of pure alcohol you consumed. In the United States, one standard drink contains 14 grams of pure alcohol. That’s equivalent to any of the following:
- Beer: 12 ounces at 5% ABV
- Wine: 5 ounces at 12% ABV
- Distilled spirits: 1.5 ounces at 40% ABV (80 proof)
If you had three standard beers, you consumed about 42 grams of alcohol. But pay attention to what “standard” actually means here. A pint of craft IPA at 7.5% ABV contains significantly more alcohol than a 12-ounce domestic lager at 5%. A generous restaurant pour of wine is often 7 or 8 ounces, not 5. Most people undercount their actual intake.
For drinks that don’t fit neatly into standard categories, you can calculate grams directly: multiply the volume of the drink in milliliters by the ABV (as a decimal) by 0.789, which is the density of ethanol in grams per milliliter. A 16-ounce (473 mL) beer at 6% ABV, for example, contains about 473 × 0.06 × 0.789 = 22.4 grams of alcohol.
A Worked Example
Say a 180-pound man has four standard drinks over three hours. Here’s how the math works:
First, convert body weight to grams: 180 pounds × 453.6 = 81,648 grams. Total alcohol consumed: 4 drinks × 14 grams = 56 grams. His r value is 0.7.
BAC = (56 / (81,648 × 0.7)) × 100 − (0.015 × 3)
BAC = (56 / 57,154) × 100 − 0.045
BAC = 0.098 − 0.045 = 0.053%
Now consider a 140-pound woman who has the same four drinks over the same three hours. Her body weight in grams is 63,504, and her r value is 0.6.
BAC = (56 / (63,504 × 0.6)) × 100 − (0.015 × 3)
BAC = (56 / 38,102) × 100 − 0.045
BAC = 0.147 − 0.045 = 0.102%
Same number of drinks, same timeframe, and one person is roughly at the legal driving limit while the other is well above it. Body weight and the distribution constant make an enormous difference.
How Your Body Eliminates Alcohol Over Time
The liver breaks down alcohol at a fairly constant rate of about 0.015 BAC per hour. That’s roughly one standard drink per hour for an average-sized person, though individual metabolism varies. This rate doesn’t speed up if you drink coffee, take a cold shower, or exercise. The only thing that lowers your BAC is time.
The 0.015 subtraction in the formula is an average. Some people metabolize alcohol slightly faster or slower depending on genetics, liver health, age, and how frequently they drink. Regular heavy drinkers often develop slightly faster elimination rates because their liver enzymes ramp up production, but this is not a reliable or safe buffer.
Why Absorption Timing Matters
Alcohol can be measured in your blood within 30 to 70 minutes after a drink, but your BAC doesn’t peak the instant you finish your last sip. If you drink quickly on an empty stomach, you’ll hit a higher peak faster. If you’ve eaten a full meal, food slows gastric emptying, which delays alcohol’s passage into the small intestine where most absorption happens. This delay gives your stomach more time to begin breaking down alcohol before it ever reaches your bloodstream, resulting in a lower and later peak concentration.
This means two people who drink the exact same amount can have very different BAC curves depending on when and what they ate. The Widmark formula doesn’t account for this, which is one reason it’s an estimate rather than a measurement.
Where the Formula Falls Short
The Widmark equation is a useful approximation, but it has meaningful error margins. The r constant of 0.7 for men and 0.6 for women represents a population average, and individual values vary widely. Research measuring actual Widmark factors in individuals found a coefficient of variation around 20% for both sexes. In men, the measured r value ranged from 0.55 to 1.2; in women, from 0.46 to 1.08. That’s a huge spread, and it means your personal result could differ substantially from the formula’s output.
Several factors the formula ignores or simplifies:
- Body composition: Two people at the same weight but with different body fat percentages will have different actual r values. The formula uses a single number for each sex.
- Food in the stomach: Eating before or while drinking can meaningfully reduce peak BAC, but there’s no food variable in the equation.
- Drink speed: Slamming four drinks in 30 minutes produces a very different BAC curve than sipping them over four hours, and the formula only partially captures this through the time variable.
- Age: Older adults often have different body water ratios and may metabolize alcohol differently than the populations originally used to derive these constants.
- Medications: Certain medications slow or interfere with alcohol metabolism in ways the formula can’t predict.
The practical takeaway is that the Widmark formula can give you a ballpark figure, but it can easily be off by 20% or more in either direction for any given individual. It should never be used to decide whether you’re safe to drive. A breathalyzer or blood test measures your actual BAC; the formula only estimates it. If there’s any doubt, the number you calculated on paper is not reliable enough to stake a legal limit or your safety on.