Legal Alcohol Limit: How BAC Is Calculated (2024)

Relevance of Blood Alcohol Concentration

A person’s BAC refers to the concentration of alcohol currently in their blood. It is typically written as a percentage representative of the ratio of alcohol to blood. Thus, a 0.08% BAC means a blood sample contains 0.08% alcohol and 99.92% blood.

BAC represents a crucial measurement, as higher concentrations of alcohol result in greater impairment of an individual’s physical and cognitive faculties. BAC levels within the following ranges produce the subsequent effects:

• 0.02 to 0.04%: A lightheaded, relaxed feeling with mild impairment

• 0.05 to 0.07%: Lowered inhibitions and affected cognition and memory

• 0.08 to 0.10%: Drowsiness and physical weakness

• 0.11 to 0.15%: Severe cognitive and physical impairment

With a BAC of roughly 0.25%, a person may black out. And at 0.30%, they could die of alcohol poisoning.

What Is the Legal Alcohol Limit?

Researchers have found that alcohol affects the human brain predictably, thus allowing states to set a universal legal drinking limit that correlates with an anticipated level of impairment for the majority of drivers.

The legal driving limit for alcohol in most states is 0.08%. The reason behind that particular number is that, at such a concentration, alcohol begins to produce the following physical impairments in the majority of users:

• Blurry vision and loss of peripheral vision

• Sleepiness

• Loss of focus

• Delayed reaction time

• Inability to judge distance and speed

• Clumsiness

• Poor judgment

Drivers above the legal alcohol limit are more than likely to be unable to control the speed and direction of their vehicles. They may fail to spot pedestrians, vehicles, or fixed objects, or even fall asleep at the wheel, thus increasing the risk of car accidents ranging from minor to fatal.

There is one state, Utah, which has a lower legal limit of 0.05%. That is the concentration level at which cognitive impairment begins although physical impairment remains mild. It is argued that drivers can still make irrational judgments that expose themselves and others to injury or death, even at the lower BAC.

Drunk Driving Laws

All states have a drunk driving law with two separate violations. If a driver commits either of them, they may face criminal charges.

First, a driver violates the law by driving while impaired. Law enforcement officers look for the following dangerous actions to identify impaired drivers:

• Weaving

• Overcorrecting

• Speeding up and slowing down

• Cutting corners

• Hitting fixed objects, like the curb

• Driving the wrong way

Second, a driver commits a violation by driving with a BAC over the legal limit, even if they’ve otherwise committed no moving violations.

Measuring Blood Alcohol Content

All states have an implied consent law that requires licensed drivers to submit to breath, urine, and blood testing. If they refuse, the state can suspend their driver’s license.

The most reliable way to calculate BAC is by taking a blood test, as it will directly measure the concentration of alcohol within a defined sample. For example, a technician may draw seven milliliters of blood for a test, and it will return a result denoting the individual is over the illegal alcohol limit if it contains at least 0.08%, or at least 0.0056 grams, of alcohol.

In most cases, however, police officers use tests that only estimate BAC due to convenience. A breath test captures a sample as you exhale and displays your BAC by estimating it from the alcohol concentration in your breath. The benefit of this test is that the device necessary for the test is portable, but its downside is its unreliability.

For example, suppose that you drank but vomited shortly thereafter. That alcohol never made it into your bloodstream, but it would still be on your breath. As such, if you were to be breath tested, you’d likely trigger a false report.

Urine tests are similarly inaccurate and imprecise. They only estimate the concentration of alcohol in your blood based on your urine, and they may provide incorrect readings based on underlying medical conditions or sloppy testing procedures.

Alcohol Legal Limit Chart

The amount of alcohol that will result in a BAC test result above the legal limit will vary based on many of the following factors:

• Weight

• Metabolism

• Whether the alcohol was consumed on an empty stomach

• Time since the last drink

• How quickly the alcohol was consumed

A standard drink contains about 14 grams of alcohol. The following quantities are roughly equal to a standard drink:

• 12 ounces of beer

• 18.75 ounces of 3.2% alcohol-by-volume beer

• 5.0 ounces of wine

• 1.5 ounces of 80-proof alcohol

• 1.0 ounces of high-proof alcohol

As for how those values translate to BAC, it’s a complicated question, as everyone’s body absorbs alcohol differently.

Nevertheless, each drink can raise a person’s BAC by 0.02% to 0.04% BAC. Therefore, if you drink quickly, you will hit the legal limit after two to four drinks. And at the same time, the liver metabolizes about one drink per hour. Thus, if you drink slowly, you can subtract 0.02% to 0.04% BAC per hour.

These figures, in turn, beg some of their own questions. How many beers is the legal limit, for instance? Three to five beers put most drivers over the limit.

What is 2 times over the legal limit of alcohol? Just five glasses of wine in one hour could put a driver at double the limit.

What is 3 times over the legal limit of alcohol? Seven or more shots in one hour could put someone at triple the legal limit.

Contact ConsumerShield for a Legal Referral

Defending yourself from a false test result that shows you were over the legal alcohol limit is an uphill battle, but an experienced lawyer can help. ConsumerShield will review your situation and refer you to a lawyer for advice and representation. Contact us for a free case review.