Montana Truck Weight and Size Limits
Legal limits for a truck in Montana without a permit: Interstate axle and gross weights, height, width, and semitrailer length, each with its official source.
Montana limits at a glance
| Limit | Montana | Source |
|---|---|---|
| Interstate gross weight | 80,000 lb | FHWA, Compilation of Existing State Truck Size and Weight Limit Laws (Report to Congress, May 2015), Appendices A and B |
| Interstate single axle | 20,000 lb | FHWA, Compilation of Existing State Truck Size and Weight Limit Laws (Report to Congress, May 2015), Appendices A and B |
| Interstate tandem axle | 34,000 lb | FHWA, Compilation of Existing State Truck Size and Weight Limit Laws (Report to Congress, May 2015), Appendices A and B |
| Width (National Network) | 102 in | 23 CFR 658.15, Width (eCFR) |
| Height | 14 ft | Mont. Code Ann. § 61-10-103 |
| Semitrailer length | 53 ft (load-carrying length) in a truck tractor-semitrailer combination | Mont. Code Ann. § 61-10-104(2)(a) |
| Grandfathered semitrailer length | 53 ft | 23 CFR part 658, Appendix B, Grandfathered Semitrailer Lengths (eCFR) |
Interstate weight limits
Federal law sets the Interstate limits at 20,000 lb on a single axle, 34,000 lb on a tandem, and 80,000 lb gross, with the bridge formula on every axle group (23 CFR 658.17). States may keep higher limits they had before the federal limits took effect ("grandfather rights").
Longer combination vehicles may run up to 131,060 lb (137,800 lb under the Montana/Alberta agreement on I-15 north of Shelby) on listed routes, meeting the bridge formula above 80,000 lb.
FHWA on Montana: Montana has a grandfather provision under Federal law (23 CFR Part 658, Appendix C) to allow vehicles to operate up to 137,800 lbs. GVW on parts of the Interstate System, provided that axle limits per the FBF are maintained. See the Routes section for a full description of the routes to which this grandfather provision applies.
Bridge formula check with Montana limits
A 5-axle tractor-trailer is filled in. Change the spacings and weights to match your rig; the limits for single axles, tandems, and gross are Montana's Interstate limits.
Your result
Montana limits off the Interstate
FHWA's summary of Montana law for trucks in regular operation (no permit), including non-Interstate highways. It reflects state law as of October 1, 2012; check the state for later changes.
- Single axle
- 20,000 lbs.
- Tandem axle
- 34,000 lbs.
- Tridem axle
- Per State weight table
- Gross weight
- 131,060 lbs. provided that any vehicle carrying a divisible load over 80,000 lbs. must comply with the FBF / 137,800 lbs. for vehicles operating the Montana/Alberta Memorandum of Understanding**
- Other
- 500 lbs. per inch of tire width on non-steering axles 61-10-107(2)(a)†
Where Montana allows more than federal limits
- Farm vehicles transporting agricultural products on non-Interstate highways within 100 miles of the harvested field may exceed State weight limits by up to 20 percent on each axle and have a tire load of up to 670 lbs. per inch width.
- Vehicles operating in Montana under a Memorandum of Understanding between Montana and Alberta, Canada. A tractor with two trailing units that uses "B-Train" connections between units is allowed up to 137,800 lbs. on eight axles. These vehicles are limited to I-15 between the Canadian Border and Shelby, Montana.
- A vehicle traveling on U.S. highway 93 from the border between Canada and the United States to 10 miles south of the border is subject to the specific maximum allowable gross weight limit provided in rules adopted by the department but is not subject to maximum gross weight limits determined by the State's bridge formula.
Height, width, and length
- Width: 102 inches. States may not set a width limit of more or less than 102 inches on the National Network (23 CFR 658.15(a)). Permits can allow wider loads.
- Height: 14 ft. There is no federal height limit; each state sets its own.
- Semitrailer length: 53 ft (load-carrying length) in a truck tractor-semitrailer combination
- Grandfathered semitrailer length: 53 ft. The minimum semitrailer length the state must allow in a tractor-semitrailer combination on the National Network, based on lengths legal in the state on December 1, 1982. Federal law also requires at least 48 ft (23 CFR 658.13(b)(1)).
Over these limits you need a permit. Montana oversize/overweight permits (406-444-7262).
Montana compared with neighboring states
| State | Gross | Single | Tandem | Height | Semitrailer |
|---|---|---|---|---|---|
| Montana | 80,000 lb | 20,000 lb | 34,000 lb | 14 ft | 53 ft |
| Idaho | 80,000 lb | 20,000 lb | 34,000 lb | 14 ft | 48 ft |
| North Dakota | 80,000 lb | 20,000 lb | 34,000 lb | 14 ft | 53 ft |
| South Dakota | 80,000 lb | 20,000 lb | 34,000 lb | 14 ft | 53 ft on the Interstate and state trun… |
| Wyoming | Pending | 20,000 lb | Pending | 14 ft | 60 ft in a truck-tractor semitrailer c… |
Sources
Source: FHWA, Compilation of Existing State Truck Size and Weight Limit Laws (Report to Congress, May 2015), Appendices A and B · Effective Oct 1, 2012 · Last verified Sep 28, 2026
Source: FHWA, Compilation of Existing State Truck Size and Weight Limit Laws (Report to Congress, May 2015), Appendix A: Montana · Effective Oct 1, 2012 · Last verified Sep 28, 2026
Source: 23 CFR 658.15, Width (eCFR) · Last verified Sep 28, 2026
Source: 23 CFR part 658, Appendix B, Grandfathered Semitrailer Lengths (eCFR) · Last verified Sep 28, 2026
Source: Mont. Code Ann. § 61-10-104(2)(a) · Last verified Sep 28, 2026
Source: Mont. Code Ann. § 61-10-103 · Last verified Sep 28, 2026
Source: FHWA, Oversize/Overweight Load Permits: State permit contacts · Last verified Sep 28, 2026
Limits change, and cities, counties, and posted bridges can set lower ones. Check with Montana before you run a load near the limits. Page data last verified Sep 28, 2026.