The Law Dictionary

Your Free Online Legal Dictionary • Featuring Black’s Law Dictionary, 2nd Ed.

How To Calculate BAC

As the best judges will remind you, driving after drinking is not illegal; driving impaired is illegal. When you are enjoying yourself, make sure you know when you have reached your limit. Here is how to calculate blood alcohol level, commonly known as BAC.

How much beer, wine or liquor can I enjoy?

Some people want to know how many drinks they can consume each hour, but it is rather difficult to calculate this. Light beers might have about 4% to 9% alcohol content with heavier craft beers having about 9% to 12% alcohol content. Wine has a wide range from white 4% or so up to port at 20%. Liquor is from 33% and upwards.

Your blood alcohol content is the percentage of your blood consisting of alcohol. When you drink, the alcohol goes quickly into your bloodstream leading to your “buzz.” This also can immediately impair your:

  • Sight
  • Motor skills
  • Thought process

The rate at which someone becomes impaired or drunken is based on gender, age, body size, food in the stomach, ethnicity, and many other factors. Certain people will not be as affected by alcohol as quickly, but all must remember that law enforcement is looking at the percentage of alcohol still in your bloodstream. Each state has its own regulations for which BAC level is acceptable.

Measuring Alcohol in Blood

The original breathalyzer was invented back in 1954 using chemical oxidation and light to measure alcohol presence. Since then the technology has been upgraded tremendously. If a police officer believes that you are under the influence of alcohol or drugs, he will give you a breathalyzer on the site. This handheld model uses electrochemical platinum fuel cells technology to measure the alcohol in your breath.

For court proceedings, the most accurate readings are preferred. Police officers taking you into custody might request that you offer a blood sample back at the station. The near-infrared spectrophotometer technology is the most accurate, but all equipment has its own error range.

The BAC measures the parts of blood, which consist of alcohol. You have heard of readings, like 0.04 and 0.08, all of these are less than 1% of your blood. For example, if you blow a BAC of 0.04, it means you have 4 parts (grams) alcohol for every 10,000 parts (grams) blood. It seems rather small, but it can easily impair your senses.

“Implied Consent”: Driver’s Preemptive Agreement to Blood Alcohol Testing

Scenario:  A driver has had a few beers or even just a couple of glasses of wine over dinner with friends.  While driving elsewhere, even on the way home, for whatever reason, bright colored lights, a “wooo, wooo” of a siren, and the driver realizes that it is a police cruiser behind that is doing this.  The police officer obviously wants the driver to pull it over and stop.  Something occurred, just not sure what.  The driver does so and shuts off the car. The police officer approaches the driver’s window, which is down because the driver knows that the police officer wants to have some type of conversation with the driver.  The police officer proceeds to tell the driver that the reason for pulling the driver over is that the car appeared to be weaving or the driver changed lanes without signaling or took a corner without signaling or whatever.  The police officer asks the driver if the driver has been drinking.  The driver says two half-glasses of wine were consumed with dinner and friends about two hours ago.  The officer asks the driver to step out of the vehicle and to do certain sobriety tests.  The officer then informs the driver that he or she is under arrest for driving while under the influence of alcohol.  The police officer handcuffs the driver, puts the driver in the back of the cruiser and it is off to the station.

At the station the driver is search and told that a breathalyzer test is required.  The driver refuses.  Oops!  Wrong answer.  The driver is released under recognizance and told that a court appearance notification will be coming in the mail.  The driver is also told that the driver’s car is being impounded as the driver cannot drive the car further.  Startled, the driver asks the police officer why this is.  The officer informs the driver that because of the driver’s refusal to take the breathalyzer, the driver’s license is now automatically suspended for one year.  Now taken very aback, the driver asks somewhat irritably what the chaos is going on.  The officer, being a calm informative servant of the community, informs the driver that when the driver obtained a license and or renewal that one of the statements on the form was that by signing the application or renewal form the driver assents to taking a chemical blood alcohol test if the driver is ever stopped for suspicion of driving while under the influence of alcohol or drugs.  Dumbfounded, the driver is now left to find a ride home, to get a lawyer, and to ponder how in chaos these events just unfolded.

Every state in the United States now has a law that is called the “implied consent” law.  As the officer so ably described the situation above, each state now makes it mandatory to accept the implied consent condition or abstain from being licensed to drive.  While is seems like an encroachment on individual liberties, each individual does have the right to agree or to disagree.  The cost of signing and agreeing to the implied consent is to not drink before driving.  It does not matter now how long ago one had alcohol.  If the chaotic breathalyzer registers “guilty”, thou art guilty.