Montana has one DUI statute with several subsections, and they fall into two groups. An impairment DUI is proved by showing your ability to drive safely was diminished. A per se DUI is proved by a test result at or above a legal limit. Prosecutors often charge both for one arrest, but a person can be convicted of only one. This page explains the difference and why it matters. It is part of our Montana DUI Guide.
One Statute, Two Kinds of Proof
All of the offenses are in MCA § 61-8-1002(1). Subsection (1)(a) is the impairment offense: driving or being in actual physical control of a vehicle "while under the influence of alcohol, any drug, or a combination of alcohol and any drug." The remaining subsections are defined by a measurement:
- (1)(b): alcohol concentration of 0.08 or more in a noncommercial vehicle.
- (1)(c): 0.04 or more in a commercial motor vehicle.
- (1)(d): a THC level of 5 ng/mL or more, not counting inactive metabolites.
- (1)(f): a listed drug at or above its blood level, without a valid prescription (added in 2025).
The under-21 offense in (1)(e) is also defined by a number, but it has its own penalty scheme and does not count as a prior DUI (MCA § 61-8-1007(3)). See under-21 DUI.
Side by Side
| Question | Impairment DUI | Per se DUI |
|---|---|---|
| Subsection | MCA § 61-8-1002(1)(a) | MCA § 61-8-1002(1)(b), (c), (d), (f) |
| What the State proves | You were under the influence: your ability to safely operate a vehicle was diminished by alcohol, a drug, or both. | A test showed a level at or above the limit, such as an alcohol concentration of 0.08. |
| Is a test result required? | No. It is evidence, but the charge can be proved without one. | Yes. The number is the offense. |
| Is proof of poor driving required? | Not as an element, but the case usually rests on driving and observations. | No. |
| Role of the 0.08 inference | A 0.08 result allows a rebuttable inference of being under the influence (§ 61-8-1002(2)(c)). | None needed. |
| First offense jail | At least 24 consecutive hours, up to 6 months (§ 61-8-1007(1)(a)(i)). | No minimum, up to 6 months (§ 61-8-1007(2)(a)(i)). |
| Second offense jail | At least 7 days, up to 1 year (§ 61-8-1007(1)(a)(ii)). | At least 5 days, up to 1 year (§ 61-8-1007(2)(a)(ii)). |
| First offense fine | $600 to $1,000. | $600 to $1,000. |
The jail figures above are for cases without a passenger under 16 and without an aggravating fact. For the full ranges, see the DUI penalties chart.
Charged Together, Convicted of One
The statute speaks to this directly: "When the same acts may establish the commission of an offense under subsection (1), a person charged with the conduct may be prosecuted for a violation of another relevant subsection under subsection (1). However, the person may be convicted of only one offense under this section" (MCA § 61-8-1002(6)).
So a citation or complaint may list (1)(a) and (1)(b) for a single stop, often as alternative counts. That is not two DUIs. It is the State keeping both routes open in case one fails. If the test is suppressed, the impairment count remains. If the jury is not convinced about impairment, the number may still carry the per se count.
Why the Difference Matters at Sentencing
The penalty statute treats the two groups separately. A first conviction under (1)(a) requires at least 24 consecutive hours in jail. A first conviction under (1)(b), (c), (d), or (f) has no minimum jail term. A second conviction is at least 7 days for impairment and at least 5 days for per se. From a third offense on, the misdemeanor ranges are the same (MCA § 61-8-1007(1)(a), (2)(a)).
Because of that gap, which subsection a plea or verdict lands on is a real point in a case, not a technicality. Where the law is the same for both: a DUI sentence cannot be deferred (MCA § 61-8-1011(4)), a first conviction brings a 6-month suspension (MCA § 61-5-208(2)(b)(i)), and both kinds of conviction count as priors (MCA § 61-8-1011(1)). See plea agreements.
No Intent Is Required for Either
Both theories share one feature that surprises people. The statute imposes absolute liability (MCA § 61-8-1002(5)), which means the State does not have to prove you knew you were impaired or over the limit, or that you meant to be. Feeling fine is not a defense to either charge.
How the Defense Differs
Against a per se charge, the work centers on the number. Was the operator certified, was the instrument within its certification period, was the sample taken within a reasonable time, and was a blood sample drawn and analyzed as the law requires (MCA §§ 61-8-1018(1)(b), 61-8-1019(1))? See challenging the breath test.
Against an impairment charge, the work centers on the observations: the driving, the video, the field sobriety tests, and other explanations for what the officer saw. A test result of 0.04 or less supports an inference that you were not under the influence of alcohol (MCA § 61-8-1002(2)(a)). See what "under the influence" means and DUI under the limit.
Challenges to the stop and the arrest apply to both, because evidence from an unlawful stop can be suppressed whichever theory the State prefers (MCA § 46-13-302).
Reading Your Own Citation
Look for the subsection after 61-8-1002. An "(a)" means impairment. A "(b)" means the 0.08 offense. If the word aggravated appears, the State is alleging an added fact such as a 0.16 result, which changes the penalties under MCA § 61-8-1007(4). See aggravated DUI.
This page explains Montana law in general as of the 2025 Montana Code Annotated. Every case turns on its own facts. It is not legal advice about your specific case.