On his website, trucking safety expert witness Lew Grill, the SAGE Corporation, offers a Bridge Formula Weights Calculator:
The Bridge Formula Weights Calculator provides a convenient way to determine the maximum allowable weight that any set of axles on a motor vehicle may carry on the Interstate highway system.
Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. This is accomplished either by spreading weight over additional axles or by increasing the distance between axles. Compliance with the Bridge Formula weight limits is determined by using the following formula:
This Bridge Formula states the maximum allowable weight equals the length between axles multiplied by the number of axles divided by the number of axles minus 1 plus the number of axles mutiplied by 12 plus 36; then multiply this product by 500.
W = the overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds.
L = the distance in feet between the outer axles of any group of two or more consecutive axles.
N = the number of axles in the group under consideration.