Driving Under The Influence of Alcohol
Blood Alcohol Concentration & Standard Drinks

Blood Alcohol Concentration: Calculating BAC & Driving Impairment

Updated Nov. 17, 2019

Driving with any amount of alcohol in your system will impair your physical skills, reaction-time, vision and ability to make appropriate judgments based on the roadway environment. With very low trace quantities of alcohol, this effect is not usually noticeable. The higher your blood alcohol concentration (BAC), the more significant the impairment will be.

Where alcohol is concerned, the safest and most sensible course of action is not to drink and drive at all, even if you are under the legal BAC limit. The legal limit is based on the BAC at which most people begin to experience noticeable impairments and has been established to make the enforcement of DUI laws possible. However, as a soon-to-be qualified driver you must realize that you can become impaired at BAC levels below this legal threshold. With so many different factors influencing how much alcohol makes it into your blood and how drunk you are as a result, driver impairment due to alcohol consumption is incredibly hard to predict.

Consider the problem:

  • Two alcoholic beverages of the same size may contain totally different amounts of alcohol.
  • Two people who have consumed the same amount of alcohol may end up with different amounts in their blood (due to the rate at which they consumed it, whether they had eaten previously and various other factors).
  • Two people with the same blood alcohol concentration may be impaired by the alcohol in different ways.

Despite these variables, we do know that intoxication and driver impairment increase in-line with blood alcohol concentration. Hence, the legal BAC limit of 0.08% was established. If you are found to be over this limit while driving, you will be charged with DUI, irrespective of how intoxicated you “feel”.

BAC and driving impairment

Sticking below the 0.08% limit does not mean that you will be sober and unimpaired while driving. Alcohol affects every person differently and can result in different levels of drunkenness depending on the circumstances. A person with a BAC of 0.01% may experience slowed reflexes, blurred vision and poor judgment equivalent to somebody with 0.10% BAC.

Keep in mind that you can and will be charged with driving under the influence at ANY blood alcohol concentration of 0.01% or above, if your behavior indicates that you may be impaired by alcohol.

Understanding BAC

Your blood alcohol level is determined by dividing the volume of alcohol detected, by the volume of blood in the sample being tested. The legal limit of 0.08% translates to:

  • 8 parts of alcohol for every 10,000 parts of blood
  • 8 grams of alcohol per every 100ml of blood

The amount of alcohol in your blood is not the same as the amount of alcohol you consumed. Your BAC is influenced by your age, weight, how much you consumed, how quickly you consumed it and whether anything else interfered with the alcohol absorption. We’ll talk about this in more detail further down. For now, just know that driving with a blood alcohol concentration of 0.08% or above is illegal in every state. Furthermore:

  • Most states set the legal BAC level at 0.04% for drivers of commercial vehicles such as large trucks, buses and rental trucks.
  • Most states have a “zero tolerance” policy for drivers under the legal drinking age of 21, which forbids them from operating a vehicle with ANY amount of alcohol in their blood.

You can find out whether these rules apply in your state, by checking the DMV website or an up-to-date copy of the driver’s handbook.

The problem with “one drink”

A person’s blood alcohol concentration is determined by far more than just the number of alcoholic drinks they have consumed. For starters, one drink may contain far more or less alcohol than another. In theory, the idea of a “standard” drink can help you to keep track of how much alcohol you have consumed. A “standard” drink is said to contain 0.6 ounces of pure ethanol, which is the equivalent of:

  • A 5-ounce glass of 12 percent wine
  • A 12-ounce bottle of 5 percent beer
  • A 1.5-ounce shot of 40 percent hard liquor

Though, thinking about “one drink” as containing a set amount of alcohol can cause more problems than it solves, as people often do not know precisely what their drinks are mixed with. Often, mixed drinks contain far more alcohol than a standard beer or glass or wine, though the sugar the contain makes them taste less alcoholic. Further compounding the problem is the fact that not all bartenders (or whoever is mixing the drink) use standard measurements of liquor.

Another issue is that not all beverages of a certain type (i.e. beer, wine or hard liquor) have the same percentage of alcohol. Most beers are around the 5 percent mark, though some can be as strong as 9 or 10 percent. This can be a huge problem, as one 5-ounce bottle of 10 percent beer contains as much alcohol as two, 5-ounce bottles of 5 percent beer.

All things considered, figuring out exactly how much alcohol you have consumed can be a challenge. Even if you do know precisely how much alcohol you have put in your body, it is impossible to accurately predict how this will affect your blood alcohol concentration or level of intoxication.

What affects BAC?

How much alcohol makes it into your blood stream is determined by the following factors:

  1. 1

    Weight.
    Heavier people tend to have more fluid in their bodies than lighter people. Fluid dilutes alcohol, resulting in a lower BAC even when the same amount of alcohol has been consumed.

  2. 2

    Body fat percentage.
    Fat contains less water than muscle, bone and other bodily tissues. A person with a higher percentage of body fat will have less fluid to dilute the alcohol they consume and a higher resulting BAC.

  3. 3

    Dehydration.
    A dehydrated person will have less fluid in their body to dilute alcohol.

  4. 4

    Gender.
    Women tend to have a lower percentage of water in their body then men and as such, often end up with higher BACs than men who have consumed the same amount of alcohol.

  5. 5

    Food.
    Having food in your stomach when you drink can slow the absorption of alcohol into your blood. This effect is minor, but it does mean that a person with an empty stomach is likely to have a higher BAC than a person who has eaten prior to drinking.

  6. 6

    Carbonated drinks.
    Fizzy alcoholic drinks or mixers such as beer, champagne and soda are thought to lead to higher blood alcohol concentrations than non-carbonated drinks of the same strength. Experts believe this is due to extra gas in the digestive tract increasing the rate of absorption.

  7. 7

    Rate of consumption.
    The time it takes you to consume a measure alcohol affects your blood alcohol concentration. Drinking a single shot of whiskey mixed in with a soda will take longer and therefore lead to a lower BAC than drinking the same shot of whiskey neat in one hit.

  8. 8

    Drugs.
    Certain illegal and prescription drugs are known to interact with alcohol, causing your body to absorb it at a faster or slower rate than normal.

In addition to the factors listed here, your age, genetic make-up, tolerance (how accustomed you are to drinking alcohol) and emotional state are all thought to influence blood alcohol concentration. With so many variables to consider, predicting how your body will respond to drinking alcohol and whether you will end up “over the limit” is extremely difficult. If there is a possibility you may be driving, it is best to avoid alcohol altogether.

Calculating your blood alcohol concentration

The most accurate way to determine your blood alcohol level is to take a breathalyzer test (or a blood test). There are other scientific methods which can help you figure out your BAC, if you know precisely how much you drank and when. The formula described below can calculate a person’s BAC, though keep in mind that the result will only be an approximation. Do not rely on this method to decide whether you are over the legal limit to drive.

To use this method, you will need the following information:

  1. Your body weight in pounds.
  2. The amount of alcohol you consumed in ounces.
  3. The number of hours that have passed since you began drinking.

You will also need to know the average rate at which blood alcohol concentration lowers, which is 0.015% per hour. Grab yourself a pen and paper or a napkin to write on and you have everything you need to calculate your current BAC.

The formula will be slightly different for males and females, due to the differences in typical male and female body composition. The “8 or 10” value in the middle of the equation below is determined by your gender. Men should use “8” while women should use “10”.

BAC = ((Alcohol in ounces x (8 or 10) / Weight in pounds) – (0.015 x time in hours)

Here is a step-by-step breakdown of how to apply this formula:

  1. Multiply the amount of alcohol you consumed by 8 or 10 (based on whether you are male or female respectively)
  2. Divide the resulting number by your weight. This is your BAC before any time has passed, we’ll come back to it in a moment.
  3. Next, multiply the time by 0.015.
  4. Then subtract this figure from the number you arrived at following stage “2”. This number is your estimated blood alcohol concentration.

Remember that the result may be inaccurate as food, carbonated drink, the amount of fluid in your body and alcohol tolerance will also influence your BAC.

BAC calculation examples

Let’s talk through a couple of examples to show you how this BAC calculation formula works in practice.

Example one

A male weighing 160 lbs drinks four standard drinks, which amounts to 2.4 ounces of alcohol (0.6 x 4 = 2.4). He began drinking at 9:00 pm and is given a breathalyzer test at 12:00 am. Will he be over the limit?

 

2.4 ounces x 8 = 19.2
19.2 / 160 lbs = 0.12 BAC (before time passes)
3 hours x 0.015 = 0.045
0.12 – 0.045 = 0.075 BAC

Providing that the man in this example did not consume most of his drinks later in the evening, this method indicates that he should not be over the legal limit of 0.08% when asked to submit to a breathalyzer test. However, as we discussed above, a whole host of other factors may influence a person’s blood alcohol concentration. A BAC of 0.075% is extremely close to the legal limit and high enough to result in impairment. If the person in this example used this calculation to determine whether they could drive safely and legally by 12:00am, this result should be high enough to encourage them NOT to take a chance on it.

Example two

A woman weighing 115 lbs consumes two standard drinks, which amounts to 1.2 ounces of alcohol. She starts drinking at 8pm and is asked to take a breathalyzer test at 9.00pm. Is she charged with DUI?

 

1.2 ounces x 10 = 12
12 / 115 lbs = 0.10 BAC
1 hour x 0.015 = 0.015
0.10 – 0.015 = 0.085 BAC

The woman in this example is 0.005% over the legal limit and will be arrested for driving under the influence. This is an important example to take note of, as many people operate under the false belief that “two drinks” is under the legal limit. For some people, it may be. Recall that the lady in this example was quite petite, weighing just 115lbs. This amounted to less total fluid in her body and a higher BAC. A person 15lbs or so heavier than her may have been legal to drive if tested at the same time, but when the margins are so close, why risk it?

Alcohol elimination rate

The alcohol elimination rate of 0.015% per hour we use in these examples is an average. Depending on a person’s metabolism, they may eliminate alcohol from their bodies at any rate from 0.013% (slower metabolism) to 0.018% (faster metabolism) per hour. As metabolism fluctuates in-line with weight and fitness level, it is incredibly difficult to accurately measure.

As alcohol elimination rate is an essential figure in the BAC calculation formula detailed above, choosing the wrong rate for yourself would lead to an inaccurate result. If you overestimate your alcohol elimination rate when working out your BAC, you may believe you are under the legal limit when in fact, you have surpassed it.

Sobering “myths”

Your alcohol elimination rate may fluctuate over weeks, months or years if your metabolism changes. However, it cannot be altered by actions you take immediately after drinking. People often buy into the idea that certain tactics will help them sober up faster. These tactics include drinking coffee, eating a meal post-drinking, doing exercise to sweat out the alcohol, or taking a shower. Understand that these “sobering-up” tactics are all MYTHS. You cannot rid your system of alcohol any faster than your natural alcohol elimination rate will allow.

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