Blood Alcohol Content (BAC) Calculator

Blood Alcohol Content, commonly abbreviated as BAC, is a measure of the concentration of ethanol in the bloodstream. It is typically expressed as a percentage: a BAC of 0.08% means that 0.08 grams of alcohol are present for every 100 milliliters of blood. BAC is the primary metric used in legal, medical, and safety contexts to assess the degree to which a person's system has been affected by alcohol consumption.

This calculator uses the Widmark formula, one of the most widely cited models in forensic toxicology, to provide an educational estimate of BAC based on the number of standard drinks consumed, body weight, biological sex, and the amount of time that has passed since drinking began. Because individual physiology varies significantly, the result should be understood as an approximation, not a clinical measurement.

The Widmark Formula

Swedish physician Erik M.P. Widmark developed his BAC estimation formula in the 1930s. The core equation is: BAC = (A / (W × r)) × 100 − 0.015 × t, where A is the total grams of pure alcohol consumed, W is body weight in kilograms, r is the Widmark factor (a sex-based distribution coefficient), and t is the number of hours elapsed since drinking began.

The Widmark factor r accounts for differences in body water content between biological males and females. On average, the male body contains more water relative to body weight, so alcohol distributes across a larger volume—giving males a lower BAC for the same amount of alcohol consumed compared to females of the same body weight. The commonly used Widmark factors are approximately 0.68 for males and 0.55 for females, though some researchers use slightly different values.

The subtracted term (0.015 × t) represents alcohol elimination. The liver metabolizes alcohol at roughly 0.015% BAC per hour, a rate that is relatively consistent across individuals though it varies somewhat by liver health, genetics, and other factors.

What Is a Standard Drink?

A key input for the Widmark formula is the amount of pure alcohol consumed, which depends on how many standard drinks you have had. In the United States, one standard drink is defined as containing 14 grams of pure ethanol. This corresponds roughly to 12 ounces (355 mL) of regular beer at approximately 5% ABV, 5 ounces (148 mL) of table wine at approximately 12% ABV, or 1.5 ounces (44 mL) of distilled spirits at approximately 40% ABV.

It is important to note that standard drink definitions differ by country. In the United Kingdom, one unit equals 8 grams of ethanol. In Japan, one standard drink is often defined as 20 grams. Many commercial drinks, particularly cocktails and craft beers, contain more than one standard drink per serving. Misjudging the number of standard drinks consumed is a common source of error when estimating BAC.

This calculator uses the US definition of 14 grams per standard drink. If you are using a different country's standard, you may need to convert or adjust your input accordingly.

How Body Weight and Sex Affect BAC

Body weight plays a significant role in BAC because alcohol distributes throughout the body's water. A person with greater body mass has more total body water and more blood volume, so the same amount of alcohol becomes more diluted. As a result, a heavier person will generally have a lower BAC than a lighter person who drinks the same amount.

Biological sex affects BAC through differences in body composition and enzyme activity. Females typically have a higher percentage of body fat relative to body weight compared to males. Since alcohol does not distribute into fat tissue, the effective volume through which alcohol dilutes is lower, producing a higher BAC per drink. Additionally, females tend to have lower levels of the stomach enzyme alcohol dehydrogenase, which is involved in first-pass metabolism of alcohol before it enters the bloodstream.

These are population-level averages. Individual variation is substantial. Factors such as age, liver health, hormonal fluctuations, use of medications, and genetics all influence how a specific person metabolizes alcohol at any given time.

The Rate of Alcohol Elimination

The liver is responsible for breaking down most of the alcohol that enters the body, processing it through enzymatic oxidation via alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). This process converts ethanol first into acetaldehyde (a toxic intermediate) and then into acetate, which is further broken down to water and carbon dioxide.

Unlike many drug elimination processes, alcohol metabolism follows zero-order kinetics over a wide range of BAC levels—meaning the liver processes alcohol at a roughly constant rate regardless of how much alcohol is in the blood. The commonly cited average elimination rate is approximately 0.015% BAC per hour, equivalent to about 7–10 grams of pure ethanol per hour for a typical adult.

This means that time is the only reliable way to lower BAC. Coffee, water, food eaten after drinking, and other commonly cited "sobering" methods do not meaningfully accelerate liver metabolism of alcohol. Food consumed before and during drinking can slow the rate of absorption from the stomach into the bloodstream, which affects peak BAC but does not speed elimination.

BAC Levels and Their General Effects

Research has documented the general range of effects associated with different BAC levels, though individual responses vary significantly based on tolerance, recent sleep, food intake, and other factors. At BAC levels below 0.01%, most people experience no measurable effects. Between roughly 0.01% and 0.039%, subtle mood changes and mild relaxation may occur.

In the range of 0.04% to 0.079%, mild sedation, reduced inhibitions, and some mild impairment to coordination and judgment may be present. Many countries set legal driving limits in or near the 0.05%–0.08% range. At 0.08% to 0.149%, coordination, reaction time, and judgment are more significantly affected. At 0.15% to 0.299%, motor control is substantially impaired, nausea may occur, and the risk of injury is elevated.

BAC levels of 0.30% and above are associated with severe impairment, loss of consciousness, respiratory depression, and risk of fatal alcohol poisoning. These effects are averages reported in the research literature and should not be used to gauge personal impairment. Individual responses differ substantially, and tolerance built over time does not reduce the physiological impairment at a given BAC—it merely reduces the perceived effect.

Limitations of BAC Estimation

The Widmark formula provides a reasonable approximation but has important limitations. It does not account for individual variation in absorption rate, which depends heavily on whether food was consumed, the speed of drinking, the type of drinks, carbonation, and gastric motility. The formula also assumes a fixed elimination rate, whereas real metabolic rates vary considerably.

The formula does not account for alcohol tolerance. A person who drinks regularly may exhibit fewer behavioral signs of impairment at a given BAC, but their actual BAC and the physiological effects on reaction time and cognitive function are not reduced proportionally. Tolerant drinkers may feel "fine" at BAC levels that substantially impair driving ability.

For these reasons, this calculator is intended solely for educational purposes. It should never be used to decide whether it is safe to drive, operate machinery, care for children, or perform any other safety-critical task. If there is any doubt about impairment, the safest course is to not drive and to arrange alternative transportation.