Federal Bridge Formula — Weight and Size Limits
23 CFR Part 658: The federal formula governing maximum allowable vehicle weight on interstate highways.
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The Federal Bridge Formula — codified at 23 CFR Part 658 — establishes the maximum allowable weights for commercial vehicles traveling on the Interstate Highway System. The formula calculates the maximum weight permitted on any group of two or more consecutive axles based on the number of axles and the distance between them. The goal is to protect bridge infrastructure by preventing excessive weight concentration.
The two most important weight limits for commercial trucks are the federal gross vehicle weight limit of 80,000 lbs on the interstate system, and the Bridge Formula limits for axle group weights. The Bridge Formula can actually allow gross weights above 80,000 lbs for vehicles with more axles spread over greater distances — so a truck that would exceed the formula on five axles might be legal on six axles with wider spacing. This is why axle configuration and spacing matter in commercial vehicle design and selection.
States can permit weights above federal limits on non-interstate routes under their own authority, and special oversize and overweight permits are available for certain loads. However, on the Interstate Highway System, the federal limits are the ceiling and no state can waive them. Overweight violations are among the most common non-safety violations in commercial trucking and carry significant fines.
Key Provisions
- Sets a maximum gross vehicle weight of 80,000 lbs on the federal Interstate Highway System.
- Bridge Formula calculates maximum axle group weights based on number of axles and spacing.
- Single axle limit: 20,000 lbs. Tandem axle limit: 34,000 lbs.
- Applies uniformly on interstate highways — states cannot waive federal weight limits on interstates.
- Special permits available for oversize and overweight loads on designated non-interstate routes.
Why It Matters to Truck Owners
Federal weight limits under the Bridge Formula directly affect what you can legally haul, how many axles your configuration needs, and the compliance risk of your fleet. Overweight violations are common at weigh stations and can result in substantial fines and required unloading on the spot. When spec’ing or buying a commercial vehicle, the axle configuration and its compliance with the Bridge Formula for your typical payload matters as much as the engine.
Frequently Asked Questions
What is the maximum weight for a commercial truck on the interstate?
The federal maximum gross vehicle weight on the Interstate Highway System is 80,000 lbs. In addition, no single axle may carry more than 20,000 lbs, no tandem axle group may carry more than 34,000 lbs, and all axle group weights must comply with the Bridge Formula based on axle spacing.
What is the Federal Bridge Formula calculation?
The Bridge Formula calculates the maximum allowable weight for any group of axles based on the number of axles (N) and the distance in feet between the outer axles of the group (L). The formula allows heavier gross weights when axles are spread further apart, which distributes the load more evenly and reduces stress on bridge structures.
Can I get a permit to exceed the 80,000 lb. federal weight limit on interstates?
No — federal law prohibits states from issuing permits for weights exceeding federal limits on the Interstate Highway System. Oversize and overweight permits are available on state routes and US highways not on the interstate system, but not on interstates where federal limits apply absolutely.
Read the official legal text: 23 CFR Part 658 — eCFR.gov (official)
Related Federal Trucking Laws
- Part 393 — Parts and Accessories for Safe Operation
- Part 390 — General Applicability and Definitions
This page is provided for informational purposes only and is not legal advice. Always verify current requirements at fmcsa.dot.gov or with a qualified transportation attorney.
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