How to calculate Bridge Formula ?

The Bridge Gross Weight Formula was developed to prevent stress to highway bridges caused by heavy trucks.
This formula provides a standard to control the spacing of truck axles and weight on those axles.

The Bridge Gross Weight per the Federal Law limits:
  • 20,000 pounds for a single axle
  • 34,000 pounds for a tandem axle group
The weight of other axle groups would be determined using the following Bridge Gross Weight Formula.
Bridge Formula

        L   =   (The spacing in feet between the outer axles of any two or more consecutive axles)

       N   =     (The number of axles being considered. 
                                          Measure the axle spacing from first to last axle in the combination)

Weight   =    lbs  (The maximum weight in pounds that can be carried on a group of 2 or more
                                           axles to the nearest 500 pounds)


Typical Loadings

Typical Loadings
Non-designated highways


1. Axles are measured to the nearest foot between the extremes on any group of two or more consecutive axles.

2. Gross weights for 5 and 6 axle vehicles apply to combinations only. For vehicles not in combination (straight trucks), only 4 axles shall be used in determining maximum gross weight.

3. Two consecutive sets of tandems may carry 34,000 pounds each providing the overall distance between the first and last axles of such consecutive sets of tandems is 36 feet or more.

4. If the distance between the centers of the first and third axles in a group of consecutive axles does not exceed 96 inches, the group is a tandem.

5. The maximum weight of any single axle is 20,000 pounds and the maximum weight of any tandem is 34,000.

6. To compensate for these weight restrictions, some tractors have a fifth wheel (also called as "slider"), that can slide forward ro backward on the tractor frame. This sliding motion shifts and balances the trailer's kingpin weight put on each axle without moving the cargo around. Sliders let you adjust the wheelbase in order to be in compliance with the bridge law.

*This document is not inteded for legal purposes.