Blood Alcohol Content (BAC)

According to most BAC charts, at a blood alcohol level of 0.08%, the drinker is over the legal driving limit and at risk of impaired motor skills that could result in accidents and death. 

Many factors — such as weight, sex, metabolism, tolerance, food eaten before drinking, medications, age, hydration, and alcohol type — can have an impact on how quickly blood alcohol concentration levels rise or fall and how long they stay in the danger zone. 

How Does Blood Alcohol Content Work? 

Blood alcohol content is a measurement of how much alcohol is present in the bloodstream, typically expressed as a percentage. BAC can be used to assess a person’s level of alcohol intoxication. 

Here’s how it works:[1,2]

Alcohol Consumption

• As soon as alcoholic beverages are consumed, the levels of alcohol in the blood begin to rise. With every drink, blood alcohol levels rise, especially if they are consumed at a rapid rate, such as more than one drink per hour.
• One “drink” is generally considered to be 5 ounces of wine, 12 ounces of beer, or 1.5 ounces of liquor. 

Metabolism of Alcohol

• The body metabolizes alcohol primarily in the liver.
• At the average rate the body metabolizes alcohol, the level of alcohol in the bloodstream will decrease by about 0.015% and 0.02% every hour.

Calculating BAC

Several tools can calculate BAC, but many organizations use the Widmark formula. In the 1920s, a researcher discovered that a single dose of alcohol in a subject weighing X kilograms produced a higher BAC than the dose divided by the person’s weight. This researcher determined the amount of water in a person’s body could raise (or lower) alcohol concentration. To allow for this issue, the researcher crafted a formula with a factor named after him to calculate BAC.

Widmark’s formula looks like the following:

BAC = [(alcohol consumed in grams) / (body weight in grams) x (the Widmark factor)] X 100

The Widmark factor depends on issues such as the person’s sex and body mass index. Typically, it’s simplified as 0.55 for females and 0.68 for males.

The result is a BAC level that is expressed as a percentage.

Let’s examine a few scenarios.

A woman weighing 74,842.7g drinks 14g of alcohol. Her Widmark calculation would look like this: [(14/74,842.7)x0.55]X100. Her BAC is 0.01.

A man weighing 77,110.7g drinks 28g of alcohol. His Widmark calculation would look like this: [(28/77,110.7)x0.68]X100. His BAC is 0.2.

To calculate your BAC, take the following steps:

1. Determine how many grams of alcohol are in each drink you take. A standard drink contains 14 g of alcohol, so you can take the number of drinks you consumed and multiply it by 14.
2. Convert your body weight from pounds to grams by multiplying the pounds by 454.
3. Use a factor of 0.55 for females and 0.68 for males.
4. Put the numbers in the formula, and do the math.

Know that the results you get are for estimation purposes only. Issues like how much you ate before you started drinking, your BMI, and your organ health can impact how quickly you metabolize alcohol.

Legal BAC Limits

• BAC limits for driving vary by country and region, but they typically top out at 0.08% in most locations. If your BAC exceeds this threshold, in legal terms, you are considered intoxicated and cannot drive safely. 
• Every state is different in terms of the repercussions for breaking drunk driving laws. In general, a drunk driver may lose their license, be required to take alcohol abuse classes, have a breathalyzer device installed in their car, and/or be incarcerated, especially for repeat offenses.

Impairment Levels

• A given BAC level will usually correlate with an expected degree of impairment. For example, a BAC of 0.08% may translate into impaired coordination and judgment, while a BAC of 0.2% or greater is considered severe and may lead to blackouts or alcohol poisoning.
• Everyone is different. For example, people with very little body mass may become intoxicated with fewer drinks than those with larger mass. Those who rarely drink will become intoxicated off fewer drinks than those who regularly drink heavily.

What Are BAC Levels Used For? 

BAC measures are used as an important indicator of alcohol intoxication levels for legal, safety, and health purposes. They can also be used to help us better understand how alcohol impacts the brain and body in differing amounts in different populations and how alcohol contributes to the development of disease, including mental health disorders. 

Here is a breakdown of how BAC levels are used:[2,4-6]

Legal Purposes

Law enforcement has access to portable breathalyzer machines that can help them to determine the BAC of a driver suspected to be under the influence. This can make it easier to protect people on the road who might otherwise become a victim of the impaired person’s driving.  

Research & Statistics

Data regarding BAC levels is collected and analyzed for research purposes, including studying the effects of alcohol on the brain and body, assessing the effectiveness of policies related to alcohol consumption, and understanding correlations between alcohol consumption and accidents, disease development, and early death.

Medical Purposes

BAC measurements can be utilized in medical settings to measure the severity of alcohol intoxication, monitor withdrawal symptoms, and inform treatment decisions for individuals suffering from alcohol-related disorders, especially in the event of an emergency admission while under the influence.

Education & Prevention

BAC testing can be an integral component of educational campaigns designed to increase public awareness about the dangers associated with alcohol use and abuse. This can help people to make more informed choices about their use of alcohol and help them to recognize when they may need help to quit drinking. 

BAC Data Chart

This chart breaks down the effects of different BAC levels:[1,5]

Can Certain Factors Impact Your BAC Levels When Drinking Alcohol?

Yes, a number of factors can have an effect on your BAC levels when drinking alcohol, including affecting how fast they rise and how long they remain at different levels. These variables impact how your body absorbs, distributes, and metabolizes alcohol, which can have an impact on how different amounts of alcohol will make you feel under different conditions.[1] 

These factors all affect BAC levels:[1,5-9]

Body Weight & Composition

Heavier individuals tend to have lower BAC levels after drinking the same amount of alcohol as individuals who have less body mass. Researchers say BAC is closely tied to the amount of water in the body. The higher the mass, the more fluid the body can hold. As a result, people who weigh more are often able to drink more than those who weigh less. 

The Widmark factor we discussed above takes weight differences into account when calculating BAC.

Sex 

Women generally have a higher BAC than men after drinking the same amount of alcohol over the same period. Women tend to have lower levels of liquid in their bodies than men, and men have higher levels of an enzyme that breaks down alcohol. Hormonal changes in women can also impact BAC. All of these factors put together typically mean women can’t drink as much or as quickly as men do.

Metabolism

Alcohol metabolism rates vary. On average, alcohol is metabolized at a rate of 0.015% to 0.02% per hour, but this rate can vary greatly depending on individual differences. Issues such as liver damage can allow alcohol to linger longer in your body than expected. 

If you keep drinking when your body hasn’t processed the last dose, your BAC can rise unexpectedly. If your body naturally produces fewer enzymes to digest alcohol, you can also have higher BAC levels than others you might know.

Rate of Consumption

Faster drinking, such as drinking more drinks in a shorter period of time (more drinks in less time) leads to a quicker rise in BAC. This makes logical sense, as the Widmark formula includes a measurement of how many grams of alcohol you’ve consumed. If you take in a lot of alcohol, your BAC will rise accordingly.

Food Intake 

Eating slows alcohol absorption and reduces peak BAC. If you drink on an empty stomach, BAC levels will rise more quickly. Researchers say people who eat high-carbohydrate meals before drinking tend to get the biggest benefit when compared to people eating high-protein meals. While eating a big meal can’t keep you sober (only abstaining from alcohol can do that), combining drinks with food can help you reduce your BAC levels.

Medications

Certain drugs and conditions can interact with alcohol, raising BAC levels more quickly. Researchers say medications like verapamil and erythromycin can impair your body’s ability to produce alcohol digestive enzymes, allowing BAC to rise very quickly.

Other medications, like benzodiazepines and opioids, can interact with alcohol in unpredictable ways and lead to fatal overdoses. Your BAC levels may be the same, but your intoxication level is not. It’s never smart to mix alcohol with any medication.

Age

As we age, our bodies produce fewer of the enzymes needed for alcohol digestion. Older people also tend to take medications that may interact with alcohol, and they are more likely to have organ damage. All of these factors mean older people tend to have higher BAC levels when they drink at the same rate as younger people.

Hydration

Dehydration increases BAC levels.

Alcohol Content

Some drinks have a higher percentage of alcohol by volume. For example, a 4-ounce glass of liquor will impact BAC levels more heavily than a 4-ounce glass of wine.

Medically Reviewed By Dr. Alison Tarlow

Dr. Alison Tarlow is a Licensed Clinical Psychologist in the States of Florida and Pennsylvania, and a Certified Addictions Professional (CAP). She has been a practicing psychologist for over 15 years. Sh... Read More