Montana DUI Laws Explained

By Javier Marín, Editor·Last reviewed September 2026·How we check this

A Montana DUI charge under Montana Code Annotated §61-8-401 triggers two parallel processes: a criminal case in district or justice court and an administrative license suspension by the Montana MVD. Both move quickly, both have separate timelines, and both have to be answered separately if you want the best possible outcome. Montana's DUI penalties escalate sharply on second offense within 5 years, making the lookback window critical.

The legal threshold

Montana's per-se DUI thresholds:

Adults 21+: 0.08% BAC.
Drivers under 21: 0.02% BAC under Montana's zero-tolerance law — any detectable alcohol triggers consequences.
Commercial drivers (CDL): 0.04% BAC.
Aggravated DUI: 0.16% BAC or higher carries enhanced minimum penalties.

Montana also charges DUI on impairment evidence below the 0.08% threshold under MCA §61-8-401 — the prosecution must prove the driver was "under the influence" of alcohol, drugs, or controlled substances regardless of measured BAC.

Implied consent and chemical testing

Montana's implied consent law requires every driver to submit to chemical testing (breath, blood, or urine) when lawfully arrested for DUI. Refusing the test triggers:

6-month license suspension for a first refusal.
1-year suspension for a second refusal within 5 years.
1-year suspension for any refusal after a prior DUI conviction.

Refusal can also be used as evidence in the criminal trial, and Montana judges often issue search warrants for blood draws when the driver refuses — meaning the refusal doesn't necessarily prevent BAC evidence from being collected.

Administrative license suspension

After a DUI arrest where the BAC was 0.08% or higher (or the driver refused testing), the arresting officer typically takes the driver's Montana driver's license and issues a temporary 5-day permit. The administrative suspension begins on day 6:

6-month suspension for a first BAC failure.
6-month suspension for a first chemical-test refusal.
1-year suspension for a second BAC failure within 5 years.
1-year suspension for a second chemical-test refusal within 5 years.

The driver has 30 days from arrest to request an administrative hearing with the Montana MVD's Driver Improvement section. The hearing is separate from any criminal proceeding and addresses three narrow questions: whether the officer had lawful cause to stop and arrest, whether the BAC was 0.08% or higher, and whether the driver lawfully took the test or refused.

First-offense criminal penalties

A first-offense DUI conviction in Montana is a misdemeanor under MCA §61-8-401, carrying:

Minimum 24 hours in jail (mandatory, cannot be suspended) — Montana's mandatory minimum is shorter than Alaska's 72 hours but firm.
$600-$1,000 fine, plus court costs that often raise the total to $1,500-$2,500.
6-month license suspension (running concurrently with administrative suspension in most cases).
Mandatory completion of the ACT (Assessment, Course, and Treatment) program through a state-approved provider.
A substance abuse evaluation and any recommended treatment.
Ignition interlock device (IID) at the court's discretion on first offense — mandatory on second and subsequent offenses.
SR-22 financial responsibility filing for 3 years — see our Montana car insurance guide.
Probation up to 1 year.

Ignition Interlock Devices (IID)

Montana courts may order an IID on first-offense DUI but mandate it on second and subsequent offenses. The device is installed on every vehicle owned or driven by the offender at the offender's expense (typically $70-$120/month plus a $100-$150 install fee). The IID prevents the vehicle from starting if the driver's breath shows alcohol at or above the device threshold (typically 0.02%). The IID also conducts rolling tests during the drive; failed rolling tests trigger reports to the MVD and may extend the IID period.

Aggravating factors

Montana penalties escalate when:

BAC 0.16% or higher — aggravated DUI with enhanced minimum penalties.
An accident with property damage or injury.
A child under 16 in the vehicle (separate child-endangerment charge under MCA §45-5-628).
Refusing a chemical test.
A prior DUI within the 5-year lookback window for misdemeanor enhancement (or 10 years for felony enhancement).
Driving with a license already suspended for DUI.

Repeat offenses

Montana's misdemeanor DUI lookback window is 5 years; the felony lookback is lifetime for fourth-or-subsequent DUI. Repeat penalties:

Second offense within 5 years: minimum 7 days jail (5 consecutive), $1,200-$2,000 fine, 1-year revocation, mandatory IID for 1 year, mandatory ACT program.
Third offense within 5 years: minimum 30 days jail, $2,500-$5,000 fine, 1-year revocation, mandatory IID, mandatory ACT and treatment.
Fourth or subsequent offense (lifetime): charged as a felony under MCA §61-8-731, up to 5 years state prison, $1,000-$10,000 fine, lifetime revocation possible.

Underage zero-tolerance

Drivers under 21 with a BAC of 0.02% or higher face an automatic license suspension under Montana's zero-tolerance law. First offense: 90-day suspension. Second offense: 6-month suspension. These are administrative actions handled through the Montana MVD's Driver Improvement section, separate from any criminal DUI charge. See our Montana junior license guide for the full underage rules.

Drugged driving

Montana's DUI statute covers driving under the influence of controlled substances — including marijuana, even when used legally — at any detectable level. The state has not adopted a per-se THC threshold; impairment evidence (drug recognition expert testimony, field sobriety tests, blood test results) is the basis for prosecution. Penalties parallel alcohol DUI, with the same suspension and treatment requirements.