Whether jail is mandatory for a Montana DUI depends on which DUI offense you are convicted of and how many prior convictions count. Most tiers carry a minimum that a judge may not suspend. A first per se DUI carries none. This page lists each minimum and explains what "mandatory" means under the statute. It is part of our Montana DUI Guide.
The Minimum Jail Term for Each Misdemeanor DUI
| Offense | Impairment DUI | Per se DUI | Aggravated DUI |
|---|---|---|---|
| First | 24 consecutive hours | None | 2 days |
| Second | 7 days | 5 days | 15 days |
| Third | 30 days | 30 days | 40 consecutive days |
Sources: MCA § 61-8-1007(1)(a) for impairment DUI, (2)(a) for per se DUI, and (4)(a) for aggravated DUI. The maximum is 6 months for a first impairment or per se DUI and 1 year for every other row. The full ranges and fines are in our DUI penalties chart.
Minimums With a Passenger Under 16
| Offense | Impairment DUI | Per se DUI | Aggravated DUI |
|---|---|---|---|
| First | 48 consecutive hours | None | 4 consecutive days |
| Second | 14 days | 10 days | 45 days |
| Third | 60 days | 60 days | 90 consecutive days |
If one or more passengers under 16 were in the vehicle, the same three subsections set these higher minimums. A first per se DUI still has no minimum, although the fine doubles.
Why the Charge Matters So Much on a First Offense
Many people are cited under more than one theory for the same stop, for example an impairment DUI and a 0.08 per se DUI. You can be convicted of only one of them for the same incident (MCA § 61-8-1002(6)). On a first offense, that choice decides whether there is a jail minimum at all: 24 consecutive hours for impairment, none for per se, and 2 days if the conviction is for an aggravated DUI.
The count of priors matters just as much. A conviction more than 10 years before the new offense does not count toward a second offense, but every prior counts once a case is a third (MCA § 61-8-1011(1)(b)). Our second and third DUI page explains the counting rules.
What "Mandatory" Means in the Statute
Each penalty subsection carries the same sentence: the mandatory minimum imprisonment term "may not be served under home arrest and may not be suspended unless the judge finds that the imposition of the imprisonment sentence will pose a risk to the person's physical or mental well-being" (MCA § 61-8-1007(1)(d), (2)(d), (4)(b)). Three things follow from that wording.
- A judge cannot suspend the minimum as a matter of leniency. A clean record, a job, or a plea agreement does not remove it.
- There is one exception, and it is about health. The judge must make a finding that jail will pose a risk to the person's physical or mental well-being. The statute does not define the risk or say what proof is needed.
- A deferred sentence is not available. A court may not defer imposition of sentence for a DUI (MCA § 61-8-1011(4)). See suspended and deferred sentences.
What Happens to Jail Time Above the Minimum
The rest of the jail sentence may be suspended for up to 1 year while the person completes a chemical dependency treatment program (MCA § 61-8-1007(1)(e)). During that time the court's conditions apply, and a violation lets the court impose the jail time it suspended. In practical terms, a sentence often reads as a longer jail term with all but the minimum suspended. What a particular judge imposes within the range is a matter of discretion and varies from case to case.
A court may also order that jail be served in another facility made available by the county and approved by the court, at the defendant's expense if the defendant is financially able (MCA § 61-8-1011(2)). And the portion above the minimum may be served on home arrest (§ 61-8-1011(3)).
Credit for Time Already Served
A person against whom a judgment of imprisonment is rendered must be allowed credit for each day of incarceration before sentencing that is directly related to the same proceedings (MCA § 46-18-403(1)(a)). If you were held after your arrest, tell your attorney exactly when you were booked and released. Jail records, not memory, are what a court relies on.
Felony DUI and Under-21 Offenses
- First felony DUI. Under the corrections option, the 13-month to 2-year Department of Corrections term may not be deferred or suspended, and there is no parole from it (MCA § 61-8-1008(1)(b)(i)). The treatment court option is an alternative. See felony DUI.
- The highest felony tier. The sentence is 5 to 25 years, and the first 5 years may not be suspended (§ 61-8-1008(4)).
- Under-21 DUI. A first or second conviction carries no minimum jail. A third or later conviction carries 24 consecutive hours to 60 days if the person is 18 or older (MCA § 61-8-1007(3)(a)).
This page explains Montana law in general as of the 2025 Montana Code Annotated. It is not legal advice about your specific case.