In 49 states, you have to blow 0.08% or higher on a breath test to automatically get a DUI; Utah sets the line at 0.05%. Commercial drivers hit the threshold at 0.04%, and drivers under 21 face zero-tolerance limits of 0.02% or lower in every state.1Legal Information Institute. Blood Alcohol Content (BAC)2Utah Legislature. Utah Code 41-6a-502 – Driving Under the Influence Those are the “per se” numbers that produce an automatic charge, but they aren’t the floor: an officer can arrest and a prosecutor can convict at any BAC if your driving shows impairment.
The Per Se BAC Limits
Every state has a “per se” threshold. Blow at or above that number on a lawful breath test and you are legally intoxicated as a matter of law, no matter how sober you feel or how well you think you were driving. For most adults over 21, that number is 0.08 grams of alcohol per 100 milliliters of blood, or the breath equivalent. Utah lowered its limit to 0.05% in 2018 and remains the only state at that level.
Two groups of drivers face much lower numbers:
- Commercial drivers blow a DUI at 0.04% while operating a commercial motor vehicle. A first offense disqualifies you from holding a commercial license for one year; a second brings a lifetime ban. Hauling hazardous materials stretches the first-offense disqualification to three years.3eCFR. 49 CFR 383.51 – Disqualification of Drivers
- Drivers under 21 are covered by zero-tolerance laws in all 50 states, with BAC limits of 0.02% or lower. In many states, any detectable alcohol is a violation.4National Highway Traffic Safety Administration. Lower BAC Limits
You Can Be Charged Below the Per Se Limit
Blowing under 0.08% doesn’t make you safe from a DUI. Every state has a separate impairment-based offense that doesn’t depend on hitting a specific number. If an officer sees you swerving, failing field sobriety tests, or otherwise driving as though alcohol is affecting you, you can be arrested and convicted at 0.05%, at 0.03%, or with no breath result at all. The per se number creates an automatic presumption of guilt once you cross it. Below it, prosecutors have to prove actual impairment, which is harder but far from impossible.
What Those Numbers Actually Mean
The breath test numbers correspond to measurable changes in how your brain and body function. For a roughly 160-pound person drinking standard drinks over an hour, the National Transportation Safety Board describes the effects this way:5National Transportation Safety Board. 0.05 BAC Safety Briefing Facts
- At 0.02% — about two drinks — visual function starts to decline and your ability to divide attention between two tasks drops. You feel fine. You’re already driving worse.
- At 0.05% — about three drinks — coordination suffers, tracking moving objects gets harder, and steering becomes less precise. Reaction time to emergencies slows noticeably.
- At 0.08% — about four drinks — concentration and short-term memory fade, speed control gets harder, and your ability to process information drops. Crash risk is dramatically elevated.
Alcohol-impaired crashes killed 12,429 people in 2023.6National Highway Traffic Safety Administration. Drunk Driving | Statistics and Resources
High BAC and Aggravated DUI
There’s a second threshold worth knowing. Most states that impose enhanced penalties for “aggravated” or “high BAC” offenses draw that line at 0.15% or 0.16%. Cross it and the consequences typically include doubled minimum fines, longer mandatory jail time, extended license suspensions, and mandatory installation of an ignition interlock device.7National Conference of State Legislatures. Increased Penalties for High Blood Alcohol Content At those levels, judgment and motor control are severely compromised, and the risk of a fatal crash is many times higher than at 0.08%.
Which Breath Test Actually Counts
Not every breath test carries the same legal weight. There are two kinds, and the difference matters.
The Roadside Preliminary Breath Test
The handheld device an officer pulls out during a traffic stop is a preliminary breath test, or PBT. It gives a rough BAC reading used to decide whether to arrest you. In most states, adults over 21 can decline the PBT without facing the automatic penalties that come with refusing a post-arrest test. The PBT result can support probable cause for arrest, but in many jurisdictions it is not admissible at trial.
The Evidentiary Breath Test at the Station
After arrest, you’ll be taken to the station or a mobile testing unit for an evidentiary breath test on a more sophisticated, regularly calibrated machine. Law enforcement uses devices approved by the National Highway Traffic Safety Administration, such as the Intoxilyzer 8000 and the Alco-Sensor IV.8US Department of Transportation. Approved Evidential Breath Measurement Devices These machines sample deep lung air, which has been in close contact with your bloodstream and reflects your actual blood alcohol level. Officers are generally required to observe you for 15 to 20 minutes beforehand to make sure you haven’t burped, vomited, or put anything in your mouth that could skew the reading.
This is the test that implied consent laws apply to, and its result is what forms the backbone of a DUI prosecution.
What Happens If You Don’t Blow
Every state has an implied consent law. By driving on public roads, you’ve already agreed to submit to chemical testing if lawfully arrested on suspicion of DUI. Refusing the evidentiary breath test doesn’t mean you walk away clean. It triggers a separate set of consequences on top of any DUI charges.
The immediate penalty is automatic license suspension. For a first refusal, suspensions typically run from six months to a year, depending on the state. Second or later refusals within a lookback period bring longer suspensions, often a year or more. These are administrative actions handled by the DMV, so they take effect even if DUI charges are later dropped or reduced.
Beyond the suspension, prosecutors in many states can tell the jury you refused, and juries tend to draw unfavorable conclusions from that. Some jurisdictions add fines, mandatory alcohol treatment, or ignition interlock requirements specifically for refusal. The U.S. Supreme Court addressed the limits of these laws in Birchfield v. North Dakota, holding that states can criminalize refusal of a warrantless breath test after a DUI arrest, but not refusal of a warrantless blood draw.9Justia. Birchfield v North Dakota
Challenging a Reading Above the Limit
A reading at or above the per se number doesn’t guarantee a conviction. Defense attorneys challenge breath results regularly, and some challenges work.
Calibration and Maintenance
Breathalyzers drift out of accuracy over time, which is why states require regular calibration. A device that was overdue, had incomplete maintenance logs, or showed a history of anomalous readings becomes vulnerable at trial. In State v. Chun, the New Jersey Supreme Court found the Alcotest device scientifically reliable, but only when specific maintenance and operational protocols were followed.10Justia. State v Jane H Chun The machine’s credibility depends on the humans maintaining it.
Rising Blood Alcohol
Alcohol doesn’t hit your bloodstream instantly. After your last drink, your BAC keeps climbing for anywhere from 30 minutes to two hours as alcohol absorbs through the stomach and small intestine. If there’s a significant gap between when you were driving and when you were tested — common after a lengthy stop, arrest, and transport — your BAC at the station may have been higher than it was behind the wheel. The rising blood alcohol defense argues that the test captured a peak you hadn’t reached while actually driving.
Medical Conditions
Gastroesophageal reflux disease and similar conditions can push alcohol vapor from the stomach into the mouth, inflating a breath reading. Diets that produce acetone, such as strict ketogenic diets, can trigger false positives on some devices. These defenses require medical documentation, but with support in the record they can work.
Procedural Failures
If the officer skipped the required observation period, failed to get a proper deep-lung sample, or didn’t document the process correctly, the defense can argue the results are unreliable and should be excluded. Courts take procedural compliance seriously. A shortcut that saved five minutes at the station can unravel the prosecution’s case.
The Short Version
If you’re over 21 and not driving commercially, 0.08% is the number that automatically makes you legally drunk anywhere in the country except Utah, where it’s 0.05%. Commercial drivers hit it at 0.04%, and anyone under 21 hits it at 0.02% or less. Beyond those numbers, 0.15% is where most states escalate to aggravated DUI. And none of these numbers are a safe harbor below — if an officer concludes you were impaired, the number on the machine isn’t what decides the case.