Commercial vehicle weight limits on the Interstate Highway System are set by federal law at 80,000 pounds gross, 20,000 pounds on any single axle, and 34,000 pounds on a tandem axle group, with a separate bridge formula that governs how weight can be spread across axles. Off the Interstate System, each state writes its own rules, and many state and local roads allow less. A truck that scales legally on I-80 can be overweight the minute it exits onto a county highway.
The Federal 80,000-Pound Gross Limit
The gross weight ceiling on Interstate travel comes from 23 U.S.C. § 127. The statute directs the Secretary of Transportation to withhold 50 percent of a state’s federal highway funding if the state refuses to allow vehicles up to 80,000 pounds on its Interstate roads, provided those vehicles have five or more axles.1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System That funding penalty is what makes the 80,000-pound number effectively uniform across all 50 states for Interstate travel.
The figure is a ceiling, not a target. A five-axle tractor-trailer still has to satisfy the axle limits and the Federal Bridge Formula, and in practice many loaded trucks operate below 80,000 pounds because one of those other constraints kicks in first.
Axle Weight Limits
Total weight is only part of the equation. Pavement damage depends heavily on how concentrated a load is at each contact point, so federal law also caps what any single axle or tandem group can carry:
- Single axle: 20,000 pounds, including enforcement tolerances.
- Tandem axle: 34,000 pounds for two consecutive axles spaced more than 40 inches and less than 96 inches apart, including enforcement tolerances.
Both figures come from the same statute as the gross weight cap, and the same funding-withholding penalty applies to states that refuse to honor them.1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System A truck can be under 80,000 pounds overall and still violate the law if too much of that weight sits on one axle. This is what catches drivers who didn’t plan load distribution carefully.
Steering axles don’t have a separate federal statutory limit in the same form. Their practical ceiling comes from tire load ratings, and most steering axle setups carry between 12,000 and 13,500 pounds depending on tire width and manufacturer rating. Exceeding a tire’s load rating is itself a violation that can put a truck out of service.
The Federal Bridge Formula
A truck can meet the gross limit and every axle limit and still be overweight under a third rule. The Federal Bridge Formula exists because bridges care about how weight is distributed along their span. Legal axle weights sitting on axles that are too close together concentrate too much load on a small section of deck.
The formula calculates the maximum allowable weight (W) on any group of two or more consecutive axles, given the distance between the outermost axles in the group (L) and the number of axles (N):2Federal Highway Administration. Oversize/Overweight Load Permits
W = 500 × [(LN / (N − 1)) + 12N + 36]
The result rounds to the nearest 500 pounds. One built-in exception matters for common tractor-trailer configurations: two consecutive sets of tandem axles can each carry 34,000 pounds as long as the overall distance between the first axle of the first set and the last axle of the second set is 36 feet or more.3Federal Highway Administration. Bridge Formula Weights Without that exception, many standard five-axle rigs couldn’t reach 80,000 pounds under the pure formula calculation.
Compliance here means knowing not just total weight and axle weights, but the exact spacing between every axle group. The same tractor can be legal with one trailer and out of formula with another.
Exemptions That Raise the Ceiling
Federal law carves out two allowances so carriers aren’t penalized for using heavier but cleaner equipment.
Vehicles powered primarily by natural gas or electric batteries may exceed the standard limits by up to 2,000 pounds on the power unit, with an absolute ceiling of 82,000 pounds gross. Natural gas fuel systems and battery packs weigh significantly more than a conventional diesel drivetrain, and without this allowance those trucks would have to haul less freight to stay legal.4Alternative Fuels Data Center. Natural Gas Vehicle (NGV) and Electric Vehicle (EV) Weight Exemption
Trucks equipped with auxiliary power units or other idle reduction technology qualify for an additional 550 pounds above the gross, single axle, tandem axle, or bridge formula limits. The driver must carry a weight certification for the equipment and be able to demonstrate that the system is functional if stopped at inspection.1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System The 550 pounds cannot be used for anything other than compensating for the weight of the idle reduction system itself.
State Limits Off the Interstate
Federal weight rules apply to the Interstate System. Everywhere else, the states are in charge. State law governs maximum weights on National Highway System routes that aren’t Interstate, as well as all state and local roads.5Federal Highway Administration. Compilation of Existing State Truck Size and Weight Limit Laws Many secondary roads and older bridges carry posted limits well below 80,000 pounds. A driver crossing state lines has to check each state’s rules independently.
Grandfathered Interstate Corridors
The 80,000-pound Interstate cap isn’t quite as universal as it sounds. Federal grandfather provisions let certain states keep higher weight limits on specific Interstate segments based on what those states allowed before the current federal standards took effect. Through statute and congressional authorization, some states permit gross weights ranging from just above 80,000 pounds to as high as 164,000 pounds on certain Interstate and National Network routes.6Federal Highway Administration. Summary of State Exceptions to Federal Truck Weight Limits Some exceptions apply only to specific corridors or turnpike routes; others cover all Interstates in the state. States also grant exemptions for particular vehicle types or commodities, though many of those apply only to non-Interstate highways.7Federal Highway Administration. Compilation of Existing State Truck Size and Weight Laws
Spring Thaw Restrictions
States in colder climates impose temporary weight reductions during spring thaw, sometimes called frost laws. When frozen ground thaws from the top down, the roadbed becomes saturated and structurally weak, and a truck that was legal in January can damage the same road in March. Restrictions typically run from late February through May, though the exact dates depend on latitude, elevation, and weather in a given year. States may cut allowable loads by 25 to 50 percent of normal limits during the restriction. Paved roads and gravel roads often operate on different schedules, and northern or higher-elevation areas tend to see longer restriction windows.
Weigh Station Enforcement
Every state operates fixed and portable weigh stations where commercial vehicles can be pulled in and scaled. Many facilities now use weigh-in-motion sensors embedded in the highway that estimate weight as a truck passes at highway speed, then flag potentially overweight vehicles for a precise check on static scales.
Carriers with strong safety records can qualify for electronic bypass programs that let their trucks pass weigh stations at highway speed. The system verifies safety credentials and checks the weigh-in-motion reading in real time. A green signal means continue; a red signal means pull in.2Federal Highway Administration. Oversize/Overweight Load Permits Bypass is a privilege, and carriers lose access if their safety records slip.
When an inspection turns up a weight violation, inspectors apply the North American Standard Out-of-Service Criteria to decide whether the truck has to be parked until the problem is fixed.8Commercial Vehicle Safety Alliance. Out-of-Service Criteria
Penalties for Overweight Violations
Fines vary widely by state, but most states calculate them on a per-pound basis above the legal limit, with the per-pound rate rising as the overload grows. A single violation can run from a few hundred dollars to several thousand depending on the state and the severity. Some states also charge the carrier for road or bridge damage caused by the overweight load, and that assessment can dwarf the fine itself.
Beyond the money, an overweight truck may be detained at the roadside until enough cargo comes off to bring it into compliance. For a divisible load, that means unloading freight on the shoulder or arranging another truck for the excess, which adds delay, reloading costs, and logistics headaches that often exceed the fine. The violation also goes on the driver’s record and the carrier’s federal file at the Federal Motor Carrier Safety Administration, where future employers and regulators can see it. Tire-related weight violations, such as exceeding a tire’s load rating, are tracked separately under FMCSA’s Safety Measurement System and affect a carrier’s Vehicle Maintenance score, which in turn increases the odds of future inspections.9Federal Motor Carrier Safety Administration. Safety Measurement System (SMS) Methodology
Overweight Permits for Non-Divisible Loads
When a load genuinely can’t be broken down to meet standard limits, carriers can apply for an overweight permit. The federal government does not issue these; every permit comes from the individual state whose roads the truck will travel.2Federal Highway Administration. Oversize/Overweight Load Permits A cross-country trip with an overweight non-divisible load may require a separate permit from every state on the route.
The word that matters is “non-divisible.” Federal regulations define a non-divisible load as one that, if separated into smaller loads, would compromise the vehicle’s function, destroy the load’s value, or require more than eight work-hours to dismantle using appropriate equipment.10Federal Highway Administration. Questions and Answers About Vehicle Size and Weight The eight-hour threshold counts total person-hours, so four workers spending two hours each would satisfy it. The carrier bears the burden of proving the load qualifies. Overweight permits for divisible loads exist in some states, but only where the state has grandfathered rights or specific congressional authorization for a particular commodity or route.
An application typically requires the vehicle configuration, load weight and dimensions, and a proposed route. States may impose travel conditions such as restricted hours, designated routes, speed limits, and escort vehicles. Single-trip permits handle unique or especially heavy loads; annual permits cover carriers that regularly move similar equipment. Single-trip fees generally run from around $10 to several hundred dollars depending on state, load, and distance. Build permit time into the schedule, because some states take days to approve complex routes, and traveling without an approved permit turns an otherwise legal shipment into a violation.