Illinois sets your legal BAC limit at 0.08% if you're 21 or older

In Illinois, you cannot legally drive with a blood alcohol concentration (BAC) of 0.08% or higher if you are 21 years old or older. This is the standard threshold across all U.S. states. If a police officer stops you and a breath test or blood test shows 0.08% BAC or above, you can be arrested for driving under the influence (DUI).

The 0.08% limit applies whether you feel impaired or not. You do not have to be visibly drunk or fail field sobriety tests — the BAC number alone is enough for a DUI charge. Illinois law treats this as a strict liability offense, meaning the state only needs to prove your BAC level, not that your driving was actually dangerous.

If you are under 21, the limit is much lower: 0.00% for drivers under 18, and 0.02% for drivers aged 18 to 20. Commercial drivers have a 0.04% limit while operating a commercial vehicle, regardless of age.

Key Takeaways

  • The legal BAC limit in Illinois is 0.08% for drivers 21 and older, measured by breath or blood test after a traffic stop.
  • Drivers under 21 face a 0.00% limit (under 18) or 0.02% limit (18–20), and commercial drivers face a 0.04% limit.
  • A BAC of 0.08% or higher can result in arrest and DUI charges even if you do not show obvious signs of impairment.
  • Illinois uses either a breathalyzer test at the roadside or a blood test at a police station to measure BAC, and you have the right to request a blood test if offered a breath test first.

How police measure BAC in Illinois

Police use two main methods to measure your BAC: a breath test (breathalyzer) or a blood test. At a traffic stop, an officer will typically offer a breath test first using a portable device or a larger machine at the police station. The breath test estimates BAC by measuring alcohol in your lungs, which correlates to alcohol in your bloodstream.

If you are arrested and taken to a police station, you have the right to request a blood test instead of or in addition to a breath test. A blood test is more precise because it directly measures the amount of alcohol in your blood. Illinois law allows you to refuse a breath test at the roadside, but refusing a test after arrest (called a "chemical test") carries separate legal penalties, including automatic license suspension.

Both tests must be administered by trained personnel using equipment that is regularly calibrated and maintained. If the equipment was not properly calibrated or the officer was not certified, the results can be challenged in court.

What happens if your BAC is between 0.05% and 0.07%

If your BAC is below 0.08% but above 0.05%, you are not automatically guilty of DUI in Illinois. However, police can still arrest you if they observe other signs of impairment: slurred speech, poor coordination, failed field sobriety tests, or erratic driving. This is called "impairment-based" DUI, and the state must prove you were unable to drive safely, not just that you had some alcohol in your system.

A BAC in this range makes an arrest more likely if the officer documents impairment. If you are charged, the prosecution will use the BAC reading as evidence alongside observations from the officer and any video from the police car or body camera.

Illinois implied consent law and refusing a test

When you drive in Illinois, you automatically consent to a chemical test (breath or blood) if police suspect DUI. This is called implied consent. If you refuse the test after arrest, your driver's license will be suspended automatically, even if you are not convicted of DUI.

A first refusal results in a one-year license suspension. A second refusal within five years results in a three-year suspension. You can request a hearing with the Illinois Secretary of State to challenge the suspension, but the burden is on you to show the officer did not have reasonable grounds to request the test.

Refusing a test does not prevent prosecution for DUI — it only prevents the state from using a BAC number as evidence. The officer's observations and any video evidence can still support a DUI charge.

DUI penalties in Illinois based on BAC level

Illinois penalties for DUI depend on your BAC level and whether you have prior convictions. A first offense with a BAC of 0.08% to 0.14% is a Class A misdemeanor, punishable by up to one year in jail and a $2,500 fine. A BAC of 0.15% or higher on a first offense is still a Class A misdemeanor but carries enhanced penalties and mandatory minimum jail time.

If you cause an accident or have a passenger under 16 in the car, penalties increase. A second DUI within five years becomes a Class A felony. A third or subsequent DUI within 10 years is a felony with mandatory minimum jail sentences.

Beyond criminal penalties, a DUI conviction results in automatic license suspension (minimum one year for a first offense), mandatory alcohol education programs, and increased car insurance rates. Some employers and professional licenses also require disclosure of a DUI conviction.

How to challenge a BAC test result in court

If you are charged with DUI, you have the right to challenge the accuracy of the BAC test. Common defenses include improper calibration of the breathalyzer, lack of officer certification, failure to observe a 15-minute waiting period before the breath test (to may support no mouth alcohol skews the result), or medical conditions that affect BAC readings.

You can request discovery of the maintenance records for the testing equipment and the officer's training certification. If the equipment was not properly maintained or the officer was not certified, the test results may be excluded from evidence. An attorney can file motions to suppress the test results or request an independent blood test for comparison.

The state must prove the test was conducted properly and the result is reliable. If they cannot, the BAC evidence may be thrown out, which significantly weakens the prosecution's case.

Frequently Asked Questions

Can I be charged with DUI if my BAC is below 0.08%?

Yes. Illinois allows DUI charges based on impairment alone, even if your BAC is below 0.08%. If police observe signs of impairment — slurred speech, poor coordination, failed sobriety tests — they can arrest you and charge you with DUI. The BAC reading is evidence, but not the only evidence.

What is the difference between a breath test and a blood test for BAC?

A breath test estimates BAC by measuring alcohol in your lungs; a blood test directly measures alcohol in your blood. Blood tests are generally more accurate, but breath tests are faster and used more often at traffic stops. You have the right to request a blood test if offered a breath test first.

Do I have to take a BAC test if police ask?

You can refuse a breath test at the roadside, but refusing a chemical test after arrest carries automatic license suspension. Refusing does not prevent DUI charges — it only removes the BAC number as evidence. The officer's observations and video can still support a conviction.

How long does alcohol stay in your system for a BAC test?

BAC peaks about 30 to 90 minutes after drinking, depending on food intake and body weight. A standard drink (12 oz beer, 5 oz wine, 1.5 oz liquor) raises BAC by roughly 0.015% in an average adult. Your body eliminates alcohol at about 0.015% per hour, but this varies by individual metabolism.

Can I get my DUI conviction removed from my record in Illinois?

DUI convictions cannot be expunged in Illinois, meaning they remain on your criminal record permanently. However, you may be able to seal the record after a waiting period if the charge was dismissed or you were acquitted. An attorney can advise whether your specific case qualifies for sealing.