Fast, reliable & cost effective Equipment Transport

You’re hauling through Big Sky Country, and the last thing you need is a surprise at a weigh station. Montana’s weight regulations might seem straightforward on paper, but there’s more to it than just staying under 80,000 pounds.

I’ve seen too many drivers get caught off guard by Montana’s unique approach to weight enforcement. Unlike most states that rely heavily on posted limits, Montana uses the federal bridge formula as its primary measuring stick. That means your legal weight changes based on your axle configuration and spacing, not just what’s written on a sign.

Here’s what you actually need to know to stay legal and avoid costly violations.

The 80,000-Pound Standard (And When It Doesn’t Apply)

Every truck in Montana follows the standard 80,000-pound gross vehicle weight limit on interstate highways. This aligns with federal regulations and covers I-90, I-15, and I-94. Simple enough.

But Montana doesn’t stop there. The state relies on the federal bridge formula, which caps maximum gross weight at 131,060 pounds depending on your axle spacing and configuration. This formula determines your actual legal weight, and it’s more restrictive than you might think.

The formula looks like this: W = 500((LN/(N-1)) + 12N + 36)

Where W is gross weight, L is wheelbase in feet, and N is number of axles.

Don’t worry about memorizing it. What matters is understanding that Montana calculates your legal weight based on how your axles are spaced, not just your total load. A five-axle truck with poorly spaced axles might only be legal at 75,000 pounds, even though the standard limit is 80,000.

For insights into managing compliance during heavy operations, understanding Montana’s approach to heavy haul transport service makes a real difference.

Single Axle Limits: The 20,000-Pound Rule

No single axle in Montana can carry more than 20,000 pounds. Period.

This includes your steer axle, and here’s where drivers mess up: your steer axle is also limited by the manufacturer’s rated capacity stamped on your tire sidewall. Whichever is lower wins. If your tires are only rated for 18,000 pounds, that’s your limit, even though Montana law technically allows 20,000.

Check your tire ratings before you load. A violation here isn’t just a fine, it’s a safety issue that can get you sidelined until you fix the problem.

Tandem and Tridem Axle Configurations

For tandem axles (two consecutive axles spaced between 40 and 96 inches apart), Montana caps the weight at 34,000 pounds total. Not per axle. Combined.

Tridem axles get a bit more breathing room at 42,000 pounds for the set of three. But again, this is total weight across all three axles, not each individual axle.

These limits exist to spread your load across the pavement. Roads wear out faster when weight concentrates in one spot, and Montana takes this seriously. The state’s freeze-thaw cycles already beat up the pavement enough without overloaded axles accelerating the damage.

When You Need an Overweight Permit

Hauling something that pushes past standard limits? Montana’s permit system exists for exactly this situation.

Overweight permits allow up to 20,000 pounds in excess axle weight, with no single axle exceeding 5,000 pounds in excess weight and no tandem axle exceeding 15,000 pounds in excess axle weight. The state requires a vehicle weight analysis for loads exceeding 10,000 pounds over legal limits.

Getting a permit isn’t just paperwork. Montana evaluates your route, checks bridge capacities, and may restrict which roads you can use. Some bridges post their own weight limits, and your permit doesn’t override those restrictions.

You’re responsible for verifying your route works before you roll. If a bridge is posted at a lower weight than your permit allows, you need to find another way.

Permit holders must also follow these rules:

• Travel at posted speed limits or slower if restricted on your permit
• Carry a printed or electronic copy of your permit in the vehicle
• Stop at all weigh stations, even with bypass technology
• Only one overweight vehicle allowed on a bridge at a time

Long Combination Vehicles: Montana’s Hidden Flexibility

Montana allows Long Combination Vehicles (LCVs) under specific conditions. For vehicles operating under the Montana/Alberta Memorandum of Understanding, the maximum allowable gross weight is 137,800 pounds. For other combinations, the maximum is 131,060 pounds.

This MOU covers trucks traveling between Montana and the Canadian Province of Alberta, primarily along I-15 from the border to Shelby. LCVs with cargo-carrying length up to 103 feet with a conventional truck within a 110-foot overall length limit are permitted on designated routes.

These aren’t common configurations, but if you’re running freight across the Canadian border, knowing these exceptions can make your route more efficient.

Weigh Stations in Montana: What You Must Know

All vehicles except passenger cars and pickup trucks under 14,000 pounds are required to stop at open Montana weigh stations. Recreational vehicles get a pass unless they’re being delivered to a distributor or dealer.

Montana operates weigh stations at key locations across the state, including:

• I-94 at Wibaux (one of the busiest in the state)
• I-90 at Billings/Laurel (eastbound and westbound)
• I-90 at Haugan (near the Idaho border)
• I-15 at Butte and Lima
• MT-200 at Clearwater Junction

Even if you receive a bypass signal from PrePass or Drivewyze, you must enter an open weigh station when pulling an oversize/overweight load (even if permitted), an overwide load greater than 9 feet, an overheight load greater than 14 feet 6 inches, or a fuel tanker.

Montana doesn’t play games with weigh station violations. The state has a reputation for strict log book enforcement, and officers will find violations if they exist. Come prepared with clean logs and current permits.

To learn how enforcement impacts safety in heavy haul operations, read Ensuring Safety and Compliance in Montana’s Heavy Haul Industry.

Special Provisions and Exceptions You Should Know

Montana offers some flexibility for specific industries and situations:

Agricultural Hauling: Trucks with custom combine special permits may operate within a 100-air-mile radius of the harvested field to the point of first unloading, and can exceed weight limitations by up to 20% per axle without penalties, as long as maximum load per inch of tire width doesn’t exceed 670 pounds. These permits don’t allow interstate travel and can’t be used to haul between commercial elevators.

Border Routes: Vehicles traveling on US-93 from the Canadian border to 10 miles south, or on Montana Highway 16 from the border to 20 miles south, follow maximum allowable gross weight limits but aren’t subject to the state bridge formula.

Tire Width Limits: Montana caps weight at 600 pounds per inch of tire width for most operations, though agricultural operations get the 670-pound exception noted above.

What Violations Actually Cost You

Montana doesn’t mess around with weight violations. Fines scale with how much you’re over, and penalties can include:

• Immediate fines based on excess weight
• Potential impoundment until weight is corrected
• Points against your CDL and CSA scores
• Company-level FMCSA violations that affect your safety rating

More importantly, an overweight violation in Montana can sideline your truck. You might need to offload cargo before you’re allowed to continue, and finding a place to unload in rural Montana isn’t always easy or cheap.

The state also uses portable weigh-in-motion systems, meaning enforcement isn’t limited to fixed weigh stations. Any vehicle over 10,000 pounds can be stopped and weighed at temporary scale locations.

Road Restrictions and Seasonal Changes

Montana posts weight restrictions on certain roads during spring thaw periods. These seasonal restrictions can drop allowable weights significantly, sometimes to 60-70% of normal limits.

County roads have their own weight restrictions separate from state highways. If your route takes you off the main corridors, verify local weight limits before you go. A county road rated for 60,000 pounds won’t care about your state permit for 100,000.

Bridge posting maps are available through Montana DOT, and checking them before planning a heavy haul route can save you from expensive detours or violations.

Montana Truck Weight Quick Reference

Category	Legal Limit	Key Details
Maximum GVW (Interstate)	80,000 lbs	Federal standard, applies to all interstate highways
Maximum GVW (Bridge Formula)	131,060 lbs	Based on axle spacing and configuration
Single Axle	20,000 lbs	Also limited by tire manufacturer rating
Tandem Axle	34,000 lbs	Combined weight, spaced 40-96 inches apart
Tridem Axle	42,000 lbs	Combined weight across three axles
LCVs (Montana/Alberta MOU)	137,800 lbs	Restricted to I-15 corridor, requires permits
Standard LCVs	131,060 lbs	Must follow federal bridge formula
Maximum Excess Axle Weight (Permit)	+5,000 lbs per axle	Maximum 20,000 lbs total excess across all axles
Weigh Station Requirement	All vehicles over 14,000 lbs GVW	Bypass programs don’t exempt oversize/overweight loads
Agricultural Tolerance	+20% per axle	Within 100 air miles, max 670 lbs per inch tire width
Tire Width Limit	600 lbs per inch	670 lbs for agricultural operations

My Take on Montana’s Approach

Montana’s weight enforcement philosophy makes sense when you understand the state’s infrastructure challenges. The freeze-thaw cycle here destroys roads faster than in warmer states, and much of Montana’s highway system crosses terrain where rebuilding damaged roads costs a fortune.

The federal bridge formula approach gives Montana flexibility to protect its bridges and roads without creating a patchwork of inconsistent local limits. Once you understand how it works, it’s actually more predictable than states with dozens of special exemptions and local variations.

But that predictability only helps if you know the rules. Too many drivers assume 80,000 pounds means 80,000 pounds everywhere, and Montana’s enforcement will teach you otherwise.

The state’s reputation for strict enforcement isn’t exaggerated. Montana DOT officers know their regulations inside and out, and they have zero tolerance for drivers who claim ignorance. Come prepared, know your weights, and have your permits squared away before you hit the road.

Montana’s permit system is straightforward compared to some states, but it requires planning. Last-minute permit applications don’t always work, especially for routes that require special bridge analysis or involve restricted roads.

Final Thoughts: Stay Legal, Stay Moving

Montana’s weight limits exist for good reasons. The state’s road infrastructure takes a beating from weather, and overweight trucks accelerate that damage exponentially. Understanding these regulations isn’t just about avoiding fines, it’s about keeping Montana’s highways functional for everyone who depends on them.

If you’re running a heavy haul transport service through Montana regularly, invest time in understanding the bridge formula and how your specific axle configurations affect your legal weight. The few hours you spend learning this will save you from expensive violations and delays.

Check your permits, verify your route, know your axle weights, and stop at open weigh stations. Montana rewards drivers who take compliance seriously and makes life difficult for those who don’t.

The roads through Montana are some of the most beautiful in the country. Keep your truck legal, and you’ll actually get to enjoy them instead of sitting at a weigh station explaining why you’re 5,000 pounds over on your rear tandem.

Frequently Asked Questions

1. What is the maximum legal weight for trucks in Montana?

On interstate highways, Montana follows the standard 80,000-pound gross vehicle weight limit. However, the actual legal weight can vary based on your axle configuration and spacing, using the federal bridge formula, which may reduce the allowed weight below 80,000 pounds.

2. How much weight can each axle carry in Montana?

• Single axle: 20,000 pounds maximum, but limited by tire manufacturer rating if lower.
  
• Tandem axles: 34,000 pounds combined.
  
• Tridem axles: 42,000 pounds combined.
  

3. Do I need a permit for overweight loads in Montana?

Yes. Overweight permits allow up to 20,000 pounds in excess axle weight, with limits per axle (5,000 lbs) and tandem axle (15,000 lbs). Permits require a route review and must comply with bridge postings and travel rules.

4. Are weigh stations mandatory in Montana?

Yes. All vehicles over 14,000 pounds GVW must stop at open weigh stations. Even with bypass programs like PrePass or Drivewyze, oversize or overweight loads, overheight/overwide loads, or fuel tankers must enter.

5. Are there special exceptions for certain trucks?

Yes. Montana offers:

• Agricultural permits allowing up to 20% extra per axle within 100 air miles of the harvest.
  
• Long Combination Vehicles under Montana/Alberta agreements with higher weight limits.
  
• Specific border route exemptions and seasonal road restrictions.