Enter the axle-group span (the distance between the extreme axles of any group of 2 or more consecutive axles) and the number of axles in the group into the Bridge Weight Calculator. The calculator will evaluate the Bridge Formula weight (the maximum allowable gross weight for that axle group under the U.S. Federal Bridge Formula). Note: the federal formula uses L in feet; this calculator accepts other distance units and converts them to feet.

Federal Bridge Formula Weight Calculator

Enter any 2 values to calculate the missing variable

Bridge Weight Formula

The following two example problems outline the steps and information needed to calculate the Bridge Formula weight (maximum allowable gross weight for an axle group) using the U.S. Federal Bridge Formula.

BW = 500* (L*N/(N-1) + 12*N + 36)

Variables:

  • BW is the Bridge Formula weight (maximum allowable gross weight for the axle group) (lbs)
  • L is the distance in feet between the extreme axles of any group of 2 or more consecutive axles (ft) 
  • N is the number of axles in the group (N ≥ 2) 

Important: In U.S. federal regulations, the final legal gross weight is also subject to other limits (for example, an 80,000 lb gross limit on the Interstate System and axle limits such as 20,000 lb for a single axle and 34,000 lb for a tandem axle, unless a specific exception applies). State and route-specific rules can further restrict allowable weights.

How to Calculate Bridge Weight?

The following steps outline how to calculate the Bridge Formula weight.

  1. First, determine the distance in feet between the extreme axles of any group of 2 or more consecutive axles (ft). 
  2. Next, determine the number of axles in the group (N). 
  3. Next, gather the formula from above = BW = 500* (L*N/(N-1) + 12*N + 36).
  4. Finally, calculate the Bridge Formula weight.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

distance between the extreme axles of any group of 2 or more consecutive axles (ft) = 50

number of axles in the group = 4

BW = 500* (L*N/(N-1) + 12*N + 36) = ?