Blood alcohol concentration (BAC) is calculated using a formula that divides the grams of pure alcohol you consumed by your body weight and a distribution factor, then subtracts the amount your body has already metabolized. The result is expressed as a percentage: a BAC of 0.08% means 0.08 grams of alcohol per 100 milliliters of blood, which is the legal limit for driving in most U.S. states.
The Core Formula
The standard method for estimating BAC is called the Widmark formula. It looks like this:
BAC = (grams of alcohol consumed ÷ (body weight in kg × distribution factor)) − (elimination rate × hours of drinking)
Each variable matters. The grams of alcohol consumed comes from what and how much you drank. In the United States, one standard drink contains 14 grams of pure alcohol, whether that’s a 12-ounce beer, a 5-ounce glass of wine, or a 1.5-ounce shot of liquor. Two beers means roughly 28 grams of alcohol entering your system.
Body weight is straightforward: a heavier person has more volume for alcohol to distribute through, so the same number of drinks produces a lower concentration. The distribution factor (sometimes called the Widmark factor or “r”) accounts for how much of your body actually absorbs and holds that alcohol. The elimination rate captures how fast your liver clears it.
Why the Distribution Factor Varies by Person
Alcohol is a small, water-soluble molecule. When it enters your bloodstream, it spreads primarily into your body’s water content rather than into fat tissue. This is why body composition, not just total weight, shapes your BAC.
Women generally have a lower proportion of body water than men of similar weight, which means the same amount of alcohol concentrates into a smaller volume of fluid and produces a higher BAC. The Widmark factor captures this difference. For men, the formula is r = 1.0181 − (0.01213 × BMI). For women, it’s r = 0.9367 − (0.01240 × BMI). A higher BMI lowers the distribution factor, which in turn raises the calculated BAC for a given amount of alcohol.
Age also plays a role. As people get older, lean body mass and total body water decrease while body fat increases. These shifts mean older adults typically reach higher blood alcohol levels from the same number of drinks they handled differently when younger.
A Worked Example
Say a 180-pound (82 kg) man with a BMI of 25 drinks three beers over two hours. Here’s how the math works:
- Grams of alcohol: 3 drinks × 14 grams = 42 grams
- Distribution factor: 1.0181 − (0.01213 × 25) = 0.715
- Elimination: 0.015% per hour × 2 hours = 0.030%
- BAC: (42 ÷ (82 × 0.715 × 1000)) × 100 − 0.030 = roughly 0.042%
That’s below the 0.08% legal limit, but impairment can begin well before that threshold. The same three beers in a 130-pound woman with a lower distribution factor could easily push BAC above 0.08%.
How Fast Your Body Eliminates Alcohol
Your liver processes alcohol at a relatively fixed rate. The average person metabolizes about 7 grams per hour, which is roughly one standard drink per hour. In BAC terms, that translates to a drop of about 0.015% per hour for most people.
However, there is a three- to four-fold range in elimination rates across the population. Genetics, liver health, how often you drink, and certain medications all influence how quickly your body clears alcohol. A person on the fast end of that range might metabolize alcohol two or three times faster than someone on the slow end. This is one reason BAC charts and online calculators can only give rough estimates.
How Food Affects the Calculation
The Widmark formula assumes all the alcohol you drank has been absorbed into your bloodstream. In reality, food in your stomach slows that absorption considerably. A meal high in carbohydrates has been shown to reduce peak BAC compared to drinking on an empty stomach. The total amount of alcohol your body eventually absorbs doesn’t change much, but the peak concentration is lower and arrives later, because your stomach releases its contents into the small intestine more gradually.
This is why drinking on an empty stomach hits harder and faster. The alcohol moves quickly into the small intestine, where most absorption happens, and your BAC spikes before your liver has had time to make a dent.
How Breathalyzers Estimate BAC
When law enforcement measures your BAC with a breath test, they aren’t directly measuring your blood. Instead, the device analyzes the alcohol concentration in a sample of your exhaled breath, then multiplies it by a fixed ratio to estimate the equivalent blood level.
In the United States, that ratio is 2,100 to 1, meaning 2,100 liters of exhaled air contain roughly the same amount of alcohol as 1 milliliter of blood. Other countries use slightly different ratios: Australia and Canada also use 2,100:1, while some European nations use 2,000:1 or 2,300:1. There is no international consensus on the exact number, because the true ratio varies from person to person and even breath to breath. Body temperature, breathing patterns, and the timing of your last drink can all shift the result slightly.
Blood draws provide a direct measurement and are considered more accurate, which is why they’re often used to confirm or challenge a breath test result in legal proceedings.
Why Calculators Are Only Estimates
Every online BAC calculator uses some version of the Widmark formula, but the inputs are approximations. You probably don’t know your exact Widmark factor, your personal elimination rate, or the precise alcohol content of each drink you had. A craft IPA might contain twice the alcohol of a light beer, but both count as “one beer” if you’re not paying attention to the label.
The formula also assumes you drank at a steady pace and that absorption is complete, neither of which is usually true. In practice, BAC rises and falls in a curve: it climbs as alcohol is absorbed, peaks sometime after your last drink (usually 30 to 90 minutes later on an empty stomach), and then gradually declines as your liver works through the backlog.
BAC charts and calculators are useful for understanding the general relationship between drinks, body size, and impairment. They are not reliable enough to tell you whether you’re safe to drive. Two people who weigh the same and drink the same amount can end up with meaningfully different BAC levels based on body composition, genetics, food intake, and how quickly they drank.