Montana DUI Laws
Last reviewed July 2026 · 5 primary sources · How we research and review these pages
Reviewed by the LegalLimit editorial team →
Standard BAC limit
0.08%
Commercial driver BAC
0.04%
Under-21 BAC
0.02%
Prior-offense lookback
10-year window then lifetime
Window applies through tier 2; lifetime count from tier 3 onward.
Montana prosecutes impaired driving as DUI under MCA Title 61, Chapter 8, Part 10 (reorganized in 2021, Ch. 498). The offense has six prongs: impairment by alcohol/drugs (§ 61-8-1002(1)(a)); BAC ≥ 0.08 (1)(b); CMV BAC ≥ 0.04 (1)(c); THC ≥ 5 ng/ml (1)(d); under-21 BAC ≥ 0.02 OR any amount of THC (1)(e); and a prohibited-substances blood-level table for amphetamine, cocaine, heroin, LSD, methamphetamine, PCP, and fentanyl (1)(f). Montana is one of the few states with a numeric THC per se threshold AND a drug-specific blood-level table. The penalty ladder runs through three tiers (first: 24 hrs–6 months, $600–$1,000; second: 7 days–1 year, $1,200–$2,000; third: 30 days–1 year, $2,500–$5,000). Prior-offense lookback (§ 61-8-1011(1)(b)) is a 10-YEAR window for escalation to a second offense, but a LIFETIME lookback (all prior convictions count, regardless of age) for a third or subsequent offense — there is no 5-year window anywhere in current Montana law. A separate 'aggravated DUI' track (§ 61-8-1001) carries enhanced § 61-8-1007(4) minimums and is triggered by ANY of: a BAC of 0.16 or more (the headline trigger, reaching high-BAC first offenders with no priors at all), a court/department ignition-interlock order, a license suspended from a prior DUI, or a refusal while already suspended. A passenger under 16 doubles the mandatory minimums and fines at every tier. A fourth+ offense is a felony under § 61-8-1008 with a LIFETIME lookback — 13 months–2 years DOC + 5-year consecutive state prison, $5,000–$10,000; repeat felony offenders face up to 25 years. The 2025 amendments added fentanyl to the drug per se table under § 61-8-1002 (Ch. 264) and updated the penalty and treatment-court provisions under §§ 61-8-1007/-1008 (Ch. 226). Montana's implied consent law deems consent to chemical testing.
Montana DUI penalties by offense tier
| Offense tier | Fine | Jail | License action | Ignition interlock |
|---|---|---|---|---|
| First offense (no prior DUI within 10 years) | $600–$1,000 | 1 day–6 months (24 consecutive hours mandatory minimum (MCA § 61-8-1007(1)(a)(i)). With passenger under 16: 48 consecutive hours and $1,200–$2,000. Non-suspendable unless the judge finds imprisonment poses a risk to the person's physical or mental well-being.; Up to 6 months (MCA § 61-8-1007(1)(a)(i)).) | Suspended for 180–365 days — License suspension governed by MCA Chapter 5 (§§ 61-5-201 et seq.). First offense typically 6-month suspension; IID-restricted license may be available. | Required if restricted license or restoration |
| Second offense within 10 years | $1,200–$2,000 | 7 days–1 year (7-day mandatory minimum. With passenger under 16: 14 days and $2,400–$4,000.) | Suspended for 365 days — 1-year license suspension per Chapter 5. | Required if restricted license or restoration |
| Third offense (lifetime lookback) OR fourth+ (felony) | $2,500–$10,000 (Third offense: $2,500–$5,000. Felony 4th+ (§ 61-8-1008): $5,000–$10,000.) | 1 month–25 years (Third offense: 30-day mandatory minimum. Felony 4th+ under § 61-8-1008(1)(a)(i): 13 months DOC placement + 5-year consecutive state prison (all suspended). Prior-sentenced 4th+: up to 10 years (2), 25 years (3), or 5–25 years non-suspendable (4).; The maximum is 25 years in state prison under § 61-8-1008(3)/(4) for repeat felony offenders.) | Revoked for 365 days — Mandatory license revocation per Chapter 5. Felony offenders face longer revocation periods. | Required |
Frequently asked questions
What is the legal BAC limit in Montana?
The per se limit is 0.08% (MCA § 61-8-1002(1)(b)). The CMV threshold is 0.04% (1)(c). Montana also has a THC per se limit of 5 ng/ml (1)(d), a drug blood-level table (1)(f), and an under-21 limit of 0.02% BAC and any-amount THC (1)(e).
What are the penalties for a first DUI in Montana?
A first offense (§ 61-8-1007(1)(a)(i)) carries 24 consecutive hours to 6 months in jail and $600–$1,000. With a passenger under 16: 48 hours to 1 year and $1,200–$2,000. Chemical dependency education and treatment are required.
What are the penalties for a second DUI in Montana?
A second offense within 10 years (§ 61-8-1011(1)(b)): 7 days to 1 year and $1,200–$2,000 (§ 61-8-1007(1)(a)(ii)) for an impairment-prong conviction, or a 5-day minimum for a per se-only conviction (§ 61-8-1007(2)(a)(ii)).
What are the penalties for a third DUI in Montana?
A third offense — using a LIFETIME lookback (§ 61-8-1011(1)(b): every prior DUI counts, regardless of age): 30 days to 1 year and $2,500–$5,000 (§ 61-8-1007(1)(a)(iii)).
How long do prior DUIs count against me in Montana?
It depends on which offense you're now charged with. Under MCA § 61-8-1011(1)(b), a prior DUI counts toward a SECOND offense only if it is less than 10 years old. For a THIRD or subsequent offense, ALL previous DUI convictions count, no matter how old — there is no 5-year window anywhere in current Montana law. So a driver with a prior DUI from 7 years ago is sentenced as a second offender (not first), and a driver with two older priors from any point in the past is sentenced as a third offender. The felony 4th+ track under § 61-8-1008 likewise uses a lifetime lookback.
When does a Montana DUI become a felony?
A fourth or subsequent offense (with 3+ qualifying convictions including DUI and specified Title 45 felonies) is a felony under § 61-8-1008, using a LIFETIME lookback. The sentence is 13 months–2 years DOC placement + 5-year consecutive state prison (all suspended), $5,000–$10,000 — or up to 5 years in a treatment court program. Repeat felony offenders face up to 25 years.
Does Montana have a THC per se limit?
Yes — Montana is one of the few states with a numeric THC per se threshold: 5 ng/ml of THC in blood (excluding inactive metabolites) under § 61-8-1002(1)(d). For drivers under 21, ANY amount of THC is prohibited under (1)(e)(ii).
What happens if I have a passenger under 16 during a Montana DUI?
A passenger under 16 doubles the mandatory minimums and fines at every tier. First offense: 48 hours (vs 24) and $1,200–$2,000 (vs $600–$1,000). Second: 14 days (vs 7) and $2,400–$4,000. Third: 60 days (vs 30) and $5,000–$10,000 (§ 61-8-1007(1)(a)).
What is Montana's under-21 rule?
Montana's under-21 offense (§ 61-8-1002(1)(e)) prohibits BAC of 0.02 or more AND any amount of THC. Penalties (§ 61-8-1007(3)): first offense $100–$500 fine; second $200–$500 + up to 10 days; third+ $300–$500 + 24 hours–60 days. License suspension 90 days / 6 months / 1 year by tier.
Sources
- MCA § 61-8-1001 — Definitions (Montana Legislature (MCA)) — Accessed July 7, 2026
- MCA § 61-8-1002 — Driving under influence (Montana Legislature (MCA)) — Accessed July 7, 2026
- MCA § 61-8-1007 — Penalty for DUI; first through third offenses (Montana Legislature (MCA)) — Accessed July 7, 2026
- MCA § 61-8-1008 — Penalty for DUI; fourth and subsequent offenses (Montana Legislature (MCA)) — Accessed July 7, 2026
- MCA § 61-8-1011 — Driving under influence; conviction defined; place of imprisonment; deferral of sentence not allowed (Montana Legislature (MCA)) — Accessed July 7, 2026