Federal law sets three ceilings that apply at the same time to any commercial truck on the Interstate Highway System: 20,000 pounds on any single axle, 34,000 pounds on a tandem axle group, and 80,000 pounds gross vehicle weight. Those DOT weight limits per axle come from 23 U.S.C. § 127 and are enforced through Federal Highway Administration regulations.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System A truck violates federal law if it exceeds any one of the three, even when the other two are fine. On top of that, every consecutive group of axles has to satisfy the Federal Bridge Formula, which frequently produces a tighter cap than the flat per-axle numbers.
The Three Federal Ceilings
Each ceiling covers a different failure mode. The single-axle cap protects pavement from point loads. The tandem cap limits how much weight can concentrate on a close pair of axles. The gross weight cap protects bridges from the total load rolling across them.
- Single axle: 20,000 pounds on any axle that is not part of a tandem or larger group.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System
- Tandem axle: 34,000 pounds on two consecutive axles spaced more than 40 inches but no more than 96 inches apart.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System
- Gross vehicle weight: 80,000 pounds for combinations of five or more axles.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System
Gross vehicle weight is everything on the wheels: chassis, engine, fuel, driver, and cargo. A tractor-semitrailer that weighs 32,000 pounds empty has 48,000 pounds of usable cargo capacity before hitting 80,000 gross.
The statute enforces these numbers through funding pressure rather than federal citations. A state that fails to enforce the limits on Interstate routes within its borders risks losing 50 percent of its federal highway apportionment.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System The actual fines land at the state level.
The Federal Bridge Formula
The per-axle caps set the outer bounds, but they are rarely the binding limit. The Federal Bridge Formula controls how much weight can sit over any span of consecutive axles. The shorter the wheelbase across a group, the less weight that group can carry, because concentrated forces are what break bridges.2Federal Highway Administration. Bridge Formula Weights
The formula is W = 500 × [(L × N) / (N − 1) + 12N + 36], where W is the maximum allowable weight in pounds (rounded to the nearest 500), L is the distance in feet between the outermost axles in the group, and N is the number of axles in the group.2Federal Highway Administration. Bridge Formula Weights You don’t run it once. You run it against every possible combination of two or more consecutive axles on the vehicle, and the lowest result binds. Most carriers work from published Bridge Formula tables rather than calculating by hand.
The Tandem Exception
Without a special allowance, the formula would keep a standard five-axle tractor-semitrailer from ever reaching 80,000 pounds gross. Federal law solves that with a specific carve-out: two consecutive sets of tandem axles may each carry the full 34,000 pounds as long as the distance between the first and last axles of those tandem sets is 36 feet or more.2Federal Highway Administration. Bridge Formula Weights That puts 68,000 pounds on the trailer and drive tandems combined, with the steering axle carrying the rest up to 80,000 gross. The dimensions of the standard 53-foot semitrailer trace directly back to that rule.
Tridem Axle Groups
A tridem is three consecutive axles. Federal law does not set a flat cap for tridems the way it does for tandems, so allowable weight comes entirely out of the Bridge Formula and depends on spacing.
At the tightest common spacing of 8 to 9 feet between outer axles, a tridem can carry about 42,000 pounds. At 20 feet of spacing the allowance rises to roughly 54,000 pounds, and at 36 feet it reaches about 70,500 pounds.3FHWA Operations – Department of Transportation. Bridge Formula Weights Carriers running tridem trailers need the exact spacing measurement to know what any given load configuration can legally weigh.
Tire Ratings and the Steering Axle
An axle can meet every federal limit and still be overweight at the tire. Federal regulations prohibit states from setting tire load limits below 500 pounds per inch of tire or tread width, which sets a floor keyed to tire size.4eCFR. 23 CFR 658.17 – Weight If the load on any tire exceeds the rating stamped on its sidewall, inspectors can place the vehicle out of service, whatever the axle scale reads.
The steering axle needs particular attention. States may not restrict steering-axle weight below 20,000 pounds or the manufacturer’s rating, whichever is lower.4eCFR. 23 CFR 658.17 – Weight The manufacturer’s number almost always controls, and many steering axles on highway tractors are rated at 12,000 to 14,000 pounds. Loading a steering axle to the federal 20,000-pound ceiling usually exceeds the manufacturer’s specification and creates a real safety problem.
Extra Weight for Alternative Fuel and Idle Reduction
Battery-electric and natural-gas trucks are heavier empty because of the powertrain, and federal law provides two targeted allowances so cleaner equipment isn’t penalized.
A vehicle powered primarily by electricity or natural gas may exceed the standard limits by up to 2,000 pounds, taking gross vehicle weight to a maximum of 82,000 pounds.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System The extra weight covers the power unit itself, not additional cargo.
Trucks equipped with an auxiliary power unit or another idle-reduction system may exceed the federal gross, axle, tandem, and bridge limits by up to 550 pounds. The increase cannot exceed the actual weight of the equipment, and the driver has to be able to demonstrate on request that the system is functional and that the extra allowance is not being used to haul cargo.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System Keeping the manufacturer’s weight documentation in the cab is the simplest way to satisfy that at an inspection.
State Roads and Grandfathered Limits
Federal caps apply to the Interstate. Every state sets its own limits on non-Interstate roads, so a load that is fully legal on I-80 can become overweight the moment it exits onto a state route. Some states allow higher weights for specific commodities like agricultural products or timber; others impose lower caps on secondary roads, local bridges, or during spring thaw.
The 80,000-pound gross weight number also has a statutory caveat. States with higher weight limits in effect before July 1, 1956, or before the Federal-Aid Highway Amendments of 1974, may keep those higher limits on Interstate segments within their borders.1Office of the Law Revision Counsel. 23 USC 127 Vehicle Weight Limitations-Interstate System A small number of states still exercise those grandfathered rights, sometimes only on specific routes rather than statewide. Route-by-route checking is the only reliable approach.
Penalties for Going Over
Fines are set and collected by the states, and they scale with the overage. A truck 1,000 to 2,000 pounds heavy might see a fine in the low hundreds. A truck 10,000 or more pounds over can draw fines above $5,000, mandatory offloading, and in some jurisdictions criminal charges. Several states fine each overweight axle separately in addition to a gross weight fine, so a single stop can produce stacked penalties. Enforcement tolerances also vary: some states allow a cushion of around 1,000 pounds before writing a citation, others start at the first pound over. The federal statute references tolerances but does not define a national number.5Federal Highway Administration. Compilation of Existing State Truck Size and Weight Limit Laws
The longer-term cost sits in the safety file. Overweight violations feed into the FMCSA’s Safety Measurement System under the Vehicle Maintenance BASIC and raise the carrier’s percentile ranking. If the violation produces an out-of-service order, it carries additional severity weighting.6FMCSA. Safety Measurement System (SMS) Methodology A carrier that climbs above the intervention threshold gets warning letters, targeted inspections, and eventually full investigations. Tire overloads are their own out-of-service condition, so a single overloaded tire can put the vehicle down at the roadside even when the axle scale looks clean.
Permits for Non-Divisible Loads
When cargo genuinely cannot be broken into smaller pieces, carriers apply for an overweight or oversize permit from each state along the route. Permits are for non-divisible loads: construction equipment, prefabricated building sections, heavy industrial machinery, and similar cargo that would be damaged if disassembled.
Requirements and fees vary. Single-trip administrative fees typically run from about $20 to $150, with mileage-based surcharges and additional fees that scale with the amount over standard. Loads well beyond the standard envelope may require pilot cars, law enforcement escorts, and travel confined to specific hours or routes. Each state issues its own permit, so a coast-to-coast heavy haul can involve a dozen or more separate applications.
Permits specify exact roads, bridge crossings, and travel windows, and any deviation converts a legal move into an unpermitted overweight violation. Carriers running heavy loads regularly use routing software that accounts for bridge weight postings, overhead clearances, and turn radii along the approved path.