Blood Alcohol Content (BAC) Calculator
Estimate your blood alcohol content given your body size and the number of drinks you’ve had.
On this page:
How to Calculate Blood Alcohol Content
Predicting your blood alcohol levels after you consume alcoholic beverages is useful for many reasons. The most obvious is to determine when you can drive or operate a motor vehicle safely without your senses being compromised.
In addition, understanding how quickly your blood alcohol levels rise and fall helps you determine how quickly certain beverages will make you “drunk” and when you will be sober again and able to function at your highest abilities.
The most accurate way to determine your blood alcohol content, or BAC, is through a blood draw. This will give the amount of alcohol in your blood per.
Another way to estimate BAC is through a breathalyzer test. This is commonly used by police officers to determine if you are intoxicated above the legal limits to operate a motor vehicle.
The maximum BAC level in all states for individuals over 21 is 0.08%. Since alcohol consumption is illegal for anyone under 21, the maximum blood alcohol content allowable for these individuals is 0%.
There is an exception to the maximum BAC level, though; the state of Utah has set its maximum BAC level to 0.05%.
One way to estimate your blood alcohol concentration without a blood test or breathalyzer test is by using the Widmark formula.
The Widmark formula in simplified form is:
BAC = alcohol consumed [grams] / body weight [grams] × r x 100
The BAC is equal to the alcohol consumed in grams divided by the body weight in grams times the gender constant r, times 100.
The gender constant r is equal to 0.68 for males and 0.55 for females.
The gender constant differs in males and females due to women having a larger percentage of fat mass, or adipose tissue.
This blood alcohol concentration will give you your blood alcohol content at that moment. If you’ve been drinking for a little while, then you can account for the time that’s passed and the amount of alcohol that has already metabolized since you started.
To do this, use the following formula:
%BAC – (time elapsed [hours] × 0.015)
Substitute the time in hours since your last drink in this formula, then subtract this from the BAC formula above.
Accuracy of Blood Alcohol Content Formulas
This next formula uses alcohol consumption and body weight in grams. Since most people in the U.S. do not use grams as a common measure, you need to convert the number of drinks you have consumed to grams. In the U.S., a standard drink is equal to about fourteen grams of alcohol.
Therefore, to convert the number of standard drinks to grams, simply multiply the number of drinks you have consumed by 14. This will give you the amount of alcohol in grams.
alcohol in grams = number of standard drinks consumed × 14
You also need to convert body weight in pounds to grams. One pound is about 454 grams.
body weight in grams = body weight in pounds × 454
Studies have shown that the r factors used may be too low in some cases. This is due to the wide range of absorption factors that take place when alcohol is metabolized in the body. This can lead to an overestimation of blood alcohol levels in both men and women.
In addition, there are a number of other individual factors that affect alcohol metabolism other than gender. For example, alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the primary enzymes involved in alcohol metabolism.
Genetics plays a role in individual variations in these genes, which affects alcohol metabolism and, consequently, alcoholism risk.
Consuming alcohol with food also impacts how quickly alcohol is absorbed. The presence of food in the stomach slows stomach emptying and can reduce the absorption of alcohol.
Therefore, while this is still one useful tool to help estimate BAC, it cannot take the place of more precise measurements such as blood draws or breathalyzer tests.
What is a Standard Drink?
When using the Widmark formula, you will need to keep track of how many alcoholic beverages you have consumed over a given period of time. This should be based on a standard drink size and alcohol content, rather than the number of bottles, cans, or glasses, since these sizes will vary.
The typical drink size varies from country to country and drink to drink, but as mentioned above, a standard drink contains approximately fourteen grams of alcohol in the U.S.
|Standard Drink Type||Size||Percent Alcohol|
|Spirits (gin, whiskey, vodka)||1.5 ounces||40%|
Not all drinks contain the “standard” amount of alcohol. Therefore, you may not be able to simply multiply the number of standard drinks by 14 to calculate the amount of alcohol in grams you have consumed.
To determine the alcohol content for these drinks, you can use the formula:
alcohol [grams] = (volume of drinks) × (AC of drinks) × 0.789 × 29.5735
These numbers are derived from the fact that one U.S. fluid ounce is about 29.5735 milliliter, and 1 ml of water is about 1 gram of weight. The number 0.789 is the density of ethanol in g/cm³
You can also use a beer volume conversion calculator to estimate the volume of drinks.
Signs of Being Drunk
Alcohol affects everyone a little differently. While the legal limit for intoxication is <0.08% to safely operate a motor vehicle (or <0.08% in Utah), there is no specific BAC that indicates that you are "drunk".
However, the higher your blood alcohol concentration rises, the more likely you will start to show signs of intoxication. This includes slurred speech, lowered inhibitions, memory difficulties, confusion, “blackouts”, and even breathing problems if the levels rise high enough.
How Much Time Do You Need To Sober Up?
Along with determining your BAC, estimating the time until all of the alcohol you have consumed leaves your system is also useful.
Interestingly, unlike the rate of absorption of alcohol, the rate at which alcohol is eliminated from the body is fairly constant. While there are some variations related to past alcohol use and food consumption, this rate is about 15 mg/100mL/h (0.015 g/dL/hr) or about 0.015% per hour. This is equivalent to about 0.25-0.30 ounces of alcohol, or about 1/2 drink per hour.
Blood Alcohol Content Calculator
As you can see, it can be a bit complicated to estimate your blood alcohol content, especially if you have consumed several drinks with different alcohol percentages.
This is where using a blood alcohol content calculator comes in handy. This will allow you to quickly estimate your blood alcohol levels to ensure that you are safely and responsibly consuming alcohol.
It is never safe to drink and drive under any circumstances.
The information and formulas on this page are for educational purposes only and are an estimate using widely accepted formulas. This is certainly not a substitute for medical consultation, legal consultation, or a blood alcohol content test or examination. This information is not legal or medical advice in any way.
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