What Is Blood Alcohol Content?

Blood alcohol content (BAC) is the amount of pure alcohol in a person’s bloodstream. If a person registers a BAC of 0.10 on a test that checks alcohol concentration levels, it means that a tenth of 1% of their blood is comprised of alcohol — or one part alcohol for every 1,000 parts of blood in the person’s bloodstream.

Many factors affect a person’s blood alcohol content. One alcoholic beverage may not exactly replicate the same BAC number in different individuals, and BAC will not accurately tell how many alcoholic beverages someone consumed.

Factors That Affect BAC Levels

When a person consumes an alcoholic beverage, the ethanol in the drink produces both the feeling a person gets with consumption and the BAC readings they will have after consuming the beverage. When consuming these beverages, many factors come into play that can produce different BAC readings, including:


When medication is ingested, it can cause physiological metabolic changes. This can be expressed in two ways: it can either slow down metabolism so that it takes longer for the alcohol to process out of the body or it can speed it up and make the alcohol process faster.

In some cases, this metabolic process can interact with the medication and either lessen or magnify the medication’s effects. These interactions can cause dangerous side effects, which is why many clinicians instruct alcohol consumption to be reduced or eliminated while certain medications are being taken.

Food Intake

Alcohol is absorbed in the gastrointestinal system. If the system is empty, the absorption rate will be higher, leading to increased BAC levels.

Conversely, if food is in the stomach, it will slow the alcohol absorption rate. It should be noted that a big meal before alcohol consumption will not decrease levels of impairment associated with alcohol consumption.

Body Weight

The more or less mass a person has, the more or less area alcohol has to travel. Those who weigh less will have higher BACs than those who weigh more if they consume the same amount of alcohol.

Differing Alcohol Content In Drinks

Another contributing factor to BAC levels is differing alcohol content in different alcoholic beverages. The alcohol content in one beer generally ranges from 4-5% alcohol, while a glass of wine is often 12% alcohol. Alcohol levels can often be found on the beverage itself or its packaging.

Tolerance Does Not Equal BAC

The National Institute of Health defines tolerance as “the diminished response to alcohol or other drugs over the course of repeated or prolonged exposure.”

Tolerance does not directly indicate BAC levels, but there is a correlation. Some people with alcohol use disorders (AUD) can function normally despite their large intake of alcohol. Those who can do this show that they have a high tolerance for alcohol, which is one of the markers of an AUD diagnosis.

Effects Of Alcohol At Various BAC Levels

Below is a detailed table of the effects in a person with normal alcohol tolerance rates at different BAC levels.

Blood Alcohol Content image

What Is A Legal BAC Level?

A person can be charged with driving under the influence (DUI) or driving while intoxicated (DWI) in the US. While the terms are often used interchangeably, they are not exactly the same. DUI means that the person is under the influence of alcohol or other substances, such as over-the-counter or illicit drugs. A BAC of 0.08% and over is when a person can be charged with a DUI.

Conversely, DWI means that a person is driving while intoxicated or impaired, and the impairment could be caused by a variety of factors including drugs, alcohol, sleepiness, etc. A person can still receive a DWI charge even if their blood alcohol content is determined to be under 0.08% as they may present as impaired at lower BAC levels or with no substance in their system at all.

Consequences of being charged with a DUI or DWI can include restrictions on or revocation of license, higher community service hours, more jail time, financial penalties, or mandatory counseling.

When Does Someone Need A BAC Test?

A blood alcohol concentration test is used to determine if a person has been drinking recently by measuring the amount of alcohol in their blood. The test can detect alcohol for up to twelve hours after a person has stopped drinking and can also determine the amount that was consumed.

There are many reasons a person might need a blood alcohol content test, including:

Legal Evidence

This can be used to determine if a person is above or below the legal limit for alcohol consumption. Law enforcement, prosecutors, and defense attorneys can all use this as court evidence to show whether a person was considered legally impaired or to determine if an individual was driving under the influence of a controlled substance.


This test could be used for pre-employment, although a urine screen is more commonly used. A BAC test can also be used if there has been a workplace accident and alcohol use is suspected.


If a person has been found guilty of an alcohol-related crime, they might need to be monitored for continued alcohol usage. This might happen if a probation officer suspects the individual to be consuming alcohol or if a counselor asks a client to submit to a test if there is suspected alcohol usage during a therapy session.

Get Help For An Alcohol Use Disorder

If you or someone you know has experienced negative or dangerous consequences due to excessive alcohol usage, help is available. Contact a treatment provider today who can help you explore your treatment options and get started on the journey to an alcohol-free future.