The Federal Bridge Formula

The US rule that fails trucks which pass every individual axle limit.

The Federal Bridge Formula (23 CFR 658.17) limits the weight on any group of consecutive axles based on how far apart they are. Your gross can be under 80,000 lb and every axle under its own limit, and a group can still be over.

W = 500 × ( L·N / (N − 1) + 12·N + 36 )

• W — max weight (lb) on the group, rounded down to 500 lb, capped at 80,000 lb
• L — distance in feet between the outermost axles of the group
• N — number of axles in the group

Tighter groups carry less. A standard 5-axle rig at 51 ft is capped near 79,500 lb — so a truck scaled at 80,000 lb gross is legal on gross and illegal on the bridge formula. LoadLegit runs this on every consecutive group automatically.

Deeper dive with worked examples: the bridge formula guide.