Last Updated on August 13, 2026 by J.E.B. Insurance Services, LLC
The Federal Bridge Formula determines the maximum weight that a group of two or more consecutive axles may carry on the Interstate Highway System. It protects bridges by considering both the number of axles and the distance between them—not merely a truck’s total gross weight.
A tractor-trailer can weigh less than 80,000 pounds and still violate an axle-group limit. Drivers, owner-operators, dispatchers, and carriers therefore need to check the steer axle, tandem groups, interior combinations, and the full outer bridge before entering the Interstate.
What the Federal Bridge Formula Does
Congress enacted the Federal Bridge Formula in 1975 to limit the weight-to-length ratio of vehicles crossing bridges. Longer axle spacing distributes weight across a greater distance. Additional axles can also distribute a load, but every relevant group must remain within its permitted weight.
The FHWA Federal Bridge Formula guide explains the calculation and provides a bridge table for common axle configurations. The formula is:
W = 500 × [(L × N ÷ (N − 1)) + 12N + 36]
W is the maximum gross weight, in pounds, allowed on the axle group under review, rounded to the nearest 500 pounds.
L is the distance, in feet, between the centers of the outermost axles in that group.
N is the number of axles in the group. The calculation applies to groups of two or more consecutive axles.
Federal Axle and Gross Weight Limits
The Federal Bridge Formula works alongside other federal limits. Under 23 CFR 658.17, the general Interstate limits are 20,000 pounds on a single axle, 34,000 pounds on a tandem axle group, and 80,000 pounds gross vehicle weight. A Bridge Formula calculation may produce a lower allowable weight for a particular axle group.
These figures do not mean every 80,000-pound combination is legal. Tire ratings, equipment ratings, state rules, route restrictions, cargo placement, and the calculated weight for each axle group can impose lower limits. The lowest applicable limit controls.
Federal Bridge Formula Example: Five-Axle Outer Group
Consider a five-axle tractor-semitrailer with 51 feet between the centers of its first and fifth axles. For the outer bridge group, N equals 5 and L equals 51.
W = 500 × [(51 × 5 ÷ 4) + 60 + 36]
The result is 79,875 pounds, which rounds to 80,000 pounds. That establishes the maximum for this axle spacing under the Federal Bridge Formula, subject to the other applicable limits. The driver must still confirm that the single axle, drive tandem, trailer tandem, tires, and equipment are within their own limits.
Interior Axle Groups Must Also Pass
Checking only the full tractor-trailer is not enough. The formula applies to every relevant group of two or more consecutive axles. For a common five-axle combination, the tractor bridge, outer bridge, and interior group from the drive axles through the trailer tandems are often the critical checks.
For example, suppose axles 2 through 5 span 35 feet. With N equal to 4 and L equal to 35, the Federal Bridge Formula produces approximately 65,333 pounds, rounded to 65,500 pounds. If those four axles actually carry 68,000 pounds, the group is overweight even when the truck’s gross weight is 80,000 pounds.
Correcting the problem may require repositioning cargo, increasing usable axle spacing, or removing weight. Sliding the trailer tandems can redistribute weight between axle groups, but it does not reduce total gross weight. Our tandem axle adjustment guide explains that process and the required locking-pin checks.
The Two-Tandem Exception
Federal law includes an important exception for two consecutive tandem axle sets. Each tandem may carry 34,000 pounds when the overall distance between the first and last axles of the two tandems is at least 36 feet. This exception matters because the standard calculation could otherwise allow less weight across that interior group.
The exception does not waive the 80,000-pound federal gross limit or other applicable requirements. Drivers should verify the actual configuration and current rules instead of relying on a remembered shortcut.
State Rules, Grandfather Rights, and Special Permits
The federal rule applies to the Interstate Highway System and reasonable access routes, but state requirements can affect other roads and specific operations. Some states retain grandfathered weight limits. States may also impose bridge postings, seasonal restrictions, tire limits, axle-spacing rules, and route-specific requirements.
States may issue special permits for qualifying nondivisible vehicles or loads without applying the ordinary axle, gross, or Federal Bridge Formula limits in the same way. A permit is not permission to use every road. It may specify the route, travel window, escorts, speed, axle configuration, or other operating conditions.
Heavy-haul operators should confirm requirements in every jurisdiction on the planned route. Our guide to oversized-load safety and route planning explains why permits, clearances, escorts, and approved routes must be settled before departure.
A Practical Bridge Formula Compliance Process
Start with an accurate scale ticket. Record the steer, drive, trailer, and gross weights. A visual estimate cannot establish compliance.
Identify every relevant consecutive axle group. Measure L from the center of the first axle to the center of the last axle in each group. Count those axles to determine N.
Calculate or consult the FHWA bridge table. Compare the permitted weight with the actual weight carried by that group. Then check the separate single-axle, tandem, gross, tire, and equipment limits.
Correct the load and verify again. Reposition cargo, adjust tandems when appropriate, or return to the shipper if weight must be removed. After any adjustment, reweigh the truck. Safe truck-and-trailer loading begins with correct cargo placement and securement.
Confirm the route. Review state requirements, bridge postings, permit conditions, and non-Interstate road limits. Keep scale tickets and permits with the trip records required by the carrier or operating authority.
Common Federal Bridge Formula Mistakes
A common mistake is checking only gross weight. Another is measuring the truck’s total wheelbase but ignoring a shorter interior group. Drivers may also assume that legal tandem weights automatically make the entire combination legal, or that moving the tandems can solve an overweight gross load.
Rounding too early can also produce the wrong result. Complete the calculation first, then round W to the nearest 500 pounds. When measurements or configurations are uncertain, use the FHWA bridge table and confirm the result with the appropriate enforcement or permitting authority.
Weight Compliance and Commercial Truck Insurance
Complying with axle and bridge limits helps reduce citations, out-of-service delays, equipment stress, and unsafe handling. It does not guarantee insurance coverage or determine a claim outcome. Coverage depends on the policy language, facts of the loss, exclusions, endorsements, and applicable law.
Owner-operators should discuss their equipment, cargo, radius, authority, and heavy-haul operations with an insurance professional who understands trucking. Review our owner operator insurance guide, or request a free commercial truck insurance quote from J.E.B. Insurance Services, LLC.
Final Check Before Entering the Interstate
The Federal Bridge Formula is one part of a complete weight-compliance review. Verify the scale ticket, calculate each critical axle group, check single and tandem limits, inspect the equipment, confirm state and route requirements, and follow every permit condition. If any measurement or weight is uncertain, stop and resolve it before the truck enters the highway.