BAC Calculator

How to Use the BAC Calculator

This blood alcohol content calculator uses the Widmark formula to estimate your BAC based on the number of standard drinks consumed, your body weight, gender, and how much time has passed since your first drink. Select your unit system, enter the number of drinks, your weight, choose your gender, and input the hours elapsed. Results update instantly as you type, showing your estimated BAC percentage, impairment status, and how long until your BAC returns to zero.

This calculator provides estimates only and should never be used to determine whether it is safe to drive or operate machinery. Individual alcohol metabolism varies widely based on genetics, food intake, medications, tolerance, and overall health. When in doubt, always err on the side of caution and do not drive after consuming any alcohol.

Understanding the Widmark Formula

The Widmark formula, developed by Swedish chemist Erik Widmark in the 1930s, is the standard method for estimating BAC. The formula is: BAC = (alcohol in grams / (body weight in grams x Widmark factor)) x 100 - (0.015 x hours elapsed). The Widmark factor accounts for the proportion of body water — 0.68 for males and 0.55 for females. One standard drink in the US contains 14 grams of pure alcohol. The body eliminates alcohol at an average rate of 0.015% BAC per hour.

BAC Levels and Impairment

At 0.02-0.04% BAC, most people feel mild relaxation and slight mood elevation. At 0.05-0.07%, judgment and coordination begin to decline noticeably. At 0.08% — the legal limit in most US states — reaction time, muscle coordination, and self-control are significantly impaired. At 0.15% and above, there is major loss of balance and a significant risk of vomiting, blackout, or loss of consciousness. BAC levels of 0.30% or higher can be life-threatening and require immediate medical attention.

Factors That Affect BAC

Many factors beyond drinks, weight, and gender affect your actual BAC. Eating food before or while drinking slows alcohol absorption. Carbonated drinks may increase absorption speed. Medications can amplify alcohol's effects. Fatigue, dehydration, and stress can increase perceived impairment at lower BAC levels. Individual enzyme levels and liver health also play significant roles in alcohol metabolism. These variables mean that calculated BAC should be treated as a rough estimate, not a precise measurement.

Frequently Asked Questions

What is BAC and how is it measured?

BAC (Blood Alcohol Concentration) is the percentage of alcohol in your bloodstream by volume. A BAC of 0.08% means there are 0.08 grams of alcohol per 100 milliliters of blood. BAC can be measured with a breathalyzer, blood test, or estimated using the Widmark formula based on drinks consumed, body weight, gender, and time.

What is the legal BAC limit for driving?

In most US states and many countries, the legal BAC limit for driving is 0.08%. However, impairment begins well below this level. Some jurisdictions have lower limits — for example, 0.05% in Utah and many European countries. For commercial drivers, the limit is typically 0.04%, and for drivers under 21, most states enforce zero tolerance.

How long does it take for BAC to reach zero?

The body metabolizes alcohol at an average rate of about 0.015% BAC per hour, regardless of body weight, gender, or how much you drank. This means it takes roughly one hour to eliminate one standard drink from your system. A BAC of 0.08% takes about 5.3 hours to return to zero.

Why does gender affect BAC?

Gender affects BAC because of differences in body water content. Men typically have about 68% body water, while women average about 55%. Since alcohol distributes through body water, women generally reach higher BAC levels than men of the same weight after consuming the same amount of alcohol.

What counts as one standard drink?

In the United States, one standard drink contains 14 grams of pure alcohol. This is equivalent to 12 ounces of regular beer at 5% ABV, 5 ounces of wine at 12% ABV, or 1.5 ounces of distilled spirits at 40% ABV (80 proof).