Federal bridge law weight limits cap commercial vehicles on the Interstate System at 80,000 pounds gross, 20,000 pounds on any single axle, and 34,000 pounds on any tandem axle, and every group of two or more consecutive axles on the vehicle must also satisfy the Federal Bridge Formula.1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System A truck that meets all three hard caps can still be illegal if an inner group of axles fails the formula. That is where most compliance problems begin.
The Three Hard Caps
Congress wrote three ceilings into 23 U.S.C. 127 that apply on the Interstate System regardless of what the Bridge Formula would otherwise allow:1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System
- 80,000 pounds gross vehicle weight for combinations of five or more axles
- 20,000 pounds on a single axle
- 34,000 pounds on a tandem axle
A tandem axle, for federal purposes, is two or more consecutive axles whose centers sit more than 40 inches but no more than 96 inches apart.2eCFR. 23 CFR 658.5 Axles spaced outside that window are treated as separate single axles, and the legal math changes accordingly.
The statute repeatedly attaches the phrase “including enforcement tolerances” to these numbers.1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System That language means the federal caps already build in room for scale variation. They are not a floor to which a state must add more slack. Loading right up to 80,000 pounds leaves nothing in reserve if the scale at the next weigh station reads slightly high.
The Federal Bridge Formula
The Bridge Formula, sometimes called Formula B, is the equation Congress uses to keep concentrated loads from overstressing bridge decks and beams:
W = 500 × [(L × N) / (N − 1) + 12N + 36]
W is the maximum allowable gross weight, rounded to the nearest 500 pounds, for any group of two or more consecutive axles. L is the distance in feet between the outermost axles of that group. N is the number of axles in the group.3Federal Highway Administration. Bridge Formula Weights
The formula rewards spreading weight out. Push axles farther apart and W goes up, because the load covers more of the span. Add axles and W goes up, because force is distributed across more contact points. Shorten the spacing or run fewer axles and the ceiling drops fast.
Why Every Axle Group Has to Check Out
Compliance is not one calculation. Every possible group of consecutive axles has to pass the formula on its own. A standard five-axle tractor-trailer shows why that matters.
Picture a rig loaded to 80,000 pounds with 12,000 on the steer and 17,000 on each of the four rear axles. The distance from axle 1 to axle 3 is 20 feet, from axle 1 to axle 5 is 51 feet, and from axle 2 to axle 5 is 35 feet. Run the numbers. Axles 1 through 3: three axles across 20 feet gives a limit of 51,000 pounds, and the actual load on that group is 46,000. Pass. Axles 1 through 5: five axles across 51 feet gives a limit of 80,000 pounds, matching the gross exactly. Pass. Axles 2 through 5: four axles across 35 feet gives a limit of 65,500 pounds, but the actual weight sitting on that group is 68,000. Fail.3Federal Highway Administration. Bridge Formula Weights
The truck’s gross weight is legal. Every individual axle is legal. The inner group is not. The fix is removing cargo or opening up the spacing between axles 2 and 5 until the formula produces a number that clears 68,000 pounds. Sliding the fifth wheel changes the split between the steer axle and the drive tandems. Sliding the trailer tandems changes spacing between axle groups. Both moves ripple through every Bridge Formula check on the vehicle, and getting one group into compliance can knock another group out. Iteration at the scale is normal.
The Two-Tandem Exception
One rule keeps standard five-axle combinations viable. Two consecutive sets of tandem axles may each carry 34,000 pounds, so long as the distance from the first axle of the leading tandem to the last axle of the trailing tandem is at least 36 feet.3Federal Highway Administration. Bridge Formula Weights Without that exception, the Bridge Formula would hold the combined weight of those four axles under 68,000 pounds at typical spacing, and the standard 80,000-pound rig would not work.
How the Limits Get Enforced
Federal bridge law is not enforced by federal officers writing tickets on the side of I-80. Section 127 works through a funding lever: a state that fails to allow vehicles meeting the standards, or fails to restrict vehicles exceeding them, risks losing 50 percent of its federal highway funding.1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System Every state runs its own program as a result, and each certifies its enforcement activity to the federal government annually, reporting citations for axle, gross, and bridge formula violations along with loads shifted or offloaded.4eCFR. 23 CFR 657.15 – Certification Content
Enforcement happens at fixed weigh stations, through mobile units with portable scales, and through weigh-in-motion sensors embedded in the pavement that flag overweight vehicles before they reach the scale house.
What a Violation Costs
Because enforcement is state-run, penalties vary. Most states use graduated fines tied to pounds of excess weight. Minor overages can run a few hundred dollars, and extreme violations can reach several thousand. Some states assess the fine per axle group that exceeds its limit, so a single stop can produce multiple penalties on the same truck.
When a truck is caught overweight with a divisible load, the load usually has to be corrected before the truck moves on. Some states make offloading mandatory by statute; others leave it to officer discretion. Corrections generally involve sliding the trailer tandems, shifting cargo inside the trailer, or transferring freight to another vehicle. States report both shifted and offloaded loads in their annual federal certification.4eCFR. 23 CFR 657.15 – Certification Content
Fines are only part of the exposure. Repeated overweight citations show up on a carrier’s safety record, drawing more roadside inspections, higher insurance costs, and attention from federal safety auditors. Carriers whose contracts depend on their safety rating feel that consequence long after the fine is paid.
Grandfathered State Limits
Some states allowed heavier vehicles before Congress adopted the current caps. The statute lets those states keep going. States may permit weights that were lawful on July 1, 1956, and, for overall gross weight on axle groups, weights lawful on the date the Federal-Aid Highway Amendments of 1974 were enacted (signed January 4, 1975).1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System
A handful of states use that authority to allow gross weights above 80,000 pounds on portions of the Interstate, or axle loads above the standard caps. A rig operating legally under one state’s grandfathered rules can become illegal the moment it crosses the state line. Route planning has to account for exactly where those allowances end.
Non-Interstate roads are a separate world. State highways, county roads, and local streets often follow their own weight tables set by state law rather than the federal Bridge Formula. A truck can be legal on the Interstate and overweight on a state route two miles down the ramp. Checking the rules for each segment of a planned route is the only reliable way to avoid trouble.
Permits for Non-Divisible Loads
Loads that cannot be easily broken up, such as construction cranes, industrial generators, or precast bridge beams, can qualify for state-issued overweight permits. The federal statute recognizes this category, allowing special permits for vehicles and loads “which cannot be easily dismantled or divided.”1Office of the Law Revision Counsel. 23 USC 127 – Vehicle Weight Limitations-Interstate System
These permits set the route, the travel dates, and sometimes time-of-day restrictions. The issuing state may require escorts, signage, or advance notice to bridge engineers along the corridor. Fees and procedures differ by state, and a multi-state move typically requires a separate permit from each state the load will cross.
The line between divisible and non-divisible matters. Boxed consumer goods are divisible because the shipment can be split between two trucks. A single 150,000-pound transformer is not. Permits are only available for non-divisible loads. A carrier caught running overweight with divisible cargo faces the full penalty structure with no permit defense.