Blood Alcohol Content (BAC) Calculator

This Blood Alcohol Content (BAC) calculator estimates a person's BAC based on their alcohol intake. If you already have a BAC before you start drinking, you can estimate it based on your drink history and add it to the current calculation. To use this calculator, simply provide your body weight, gender, time, and the amount of alcoholic consumed, then click the 'Calculate' button. The result will also provide the time needed for the BAC to drop to 0 or different legal limits. Click here to find the drunk driving BAC limit for your country. This calculation is based on a model developed by Swedish professor Erik Widmark in the 1920s. Due to its limitations, the calculation results are only estimates and are not meant to be precise. Some studies have found that the real BAC of 72% of people is within ±15% of the calculation result.

Blood Alcohol Content, commonly known as BAC, measures the amount of alcohol in a person's bloodstream. It is usually expressed as a percentage. For example, a BAC of 0.08% means there are 0.08 grams of alcohol per 100 milliliters of blood. This measure helps to understand how intoxicated a person is and can be crucial for both medical and legal reasons.

BAC is important because it directly affects a person's ability to perform tasks, particularly those that require concentration and coordination, such as driving. Higher BAC levels can lead to impaired judgment, slower reaction times, and decreased motor skills, significantly increasing the risk of accidents and injuries. Most countries have legal BAC limits for drivers to ensure safety on the roads. In the United States, for example, the legal limit for drivers over 21 years old is typically 0.08% and 0.04% for commercial drivers. Wikipedia has a summary of the legal BAC limits for driving in different countries.

How is BAC Measured?

BAC can be measured in several ways, the most common methods being breath, blood, and urine tests.

Blood Tests: Blood tests provide the most accurate measure of BAC. A blood sample is taken and analyzed to determine the exact amount of alcohol present. This method is often used in medical settings or when precise measurements are required.

Breath Tests: Breathalyzers are commonly used by law enforcement to measure BAC quickly. These devices estimate BAC by measuring the amount of alcohol in the breath. Although convenient, they may not always be as accurate as blood tests.

Urine Tests: Urine tests can also be used to measure BAC, but they are less accurate than blood or breath tests. These tests measure the amount of alcohol that has passed through the body and can provide an estimate of BAC.

How is BAC Calculated?

For most people, direct BAC measurements are not easily accessible in moments like making decisions about driving. Over time, various formulas have been developed to quickly estimate BAC levels based on the amount of alcohol consumed, as well as other factors such as body weight, gender, and the time over which the alcohol was consumed. Among these, the formula developed by Swedish professor Erik Widmark in the 1920s is the most widely adopted, and it is expressed as follows:

BAC =

10 × V_d

– βT

Where:

BAC is the estimated blood alcohol content in percentage.

A is the mass of alcohol consumed (g).

T is the amount of time during which alcohol was present in the blood (usually time since consumption began), in hours.

β is the rate at which alcohol is eliminated, averaging around 0.015% per hour.

V_d is the volume of distribution (L); typically body weight (kg) multiplied by 0.71 L/kg for men and 0.58 L/kg for women.

Example: John, a man weighing 180 pounds, drank four cups of beer, each 330 ml (each containing 5% alcohol), over a period of 3 hours. What is his BAC after the drink?

Total alcohol consumed (A):

Each beer contains 13 grams of alcohol (330 ml × 0.05 × 0.789 g/ml = 13 g).
John drank four beers: 4 × 13 = 52 grams of alcohol.

John's weight (W): 180 pounds = 81.6 kg.

Volume of distribution (Vd): 81.6 kg × 0.71 L/kg = 57.9 L.

BAC =

10 × V_d

– βT

10 × 57.9

– 0.015 × 3

0.045%

So, John's estimated BAC is approximately 0.045%.

Understanding BAC is crucial for making informed decisions about alcohol consumption and ensuring safety, particularly when it comes to activities like driving. Our calculator, based on the Widmark Formula, can quickly estimate BAC levels using simple information. However, besides the factors included in the Widmark formula—such as body weight, gender, time, and alcohol consumption amount—many other factors can influence a person's BAC.

For example:

Rate of Consumption: Drinking alcohol quickly can lead to higher BAC levels, as the body has less time to metabolize it.
Food Intake: Eating food before or while drinking can slow the absorption of alcohol into the bloodstream, leading to a lower BAC.
Age: As people age, their bodies generally become less efficient at metabolizing alcohol. Older adults may experience higher BAC levels and prolonged effects from the same amount of alcohol compared to younger individuals.

All these factors not included in the Widmark Formula contribute to inaccuracies in the calculation. The Widmark Formula is most accurate when used to estimate BAC a few hours after drinking a single dose of alcohol in a fasted state, and it can be within ±20% of the true value. It is not accurate for BAC levels below 0.02% or when alcohol is consumed with food. When using the calculation result, please keep the possible error range in mind.