What the formula is for
The Federal Bridge Formula relates the number of axles in a group and the distance between the outer axles to a calculated maximum weight for that group under federal rules. It is intended to protect bridges by accounting for how concentrated a load is.
The variables
TowAtlas uses W = 500 × ((L × N)/(N − 1) + 12N + 36), where W is the calculated weight in pounds, L is the distance in feet between the outer axles of the group, and N is the number of axles in that group.
Why axle spacing matters
For the same number of axles, more spacing generally allows the formula to produce a higher result because the load is spread over a longer portion of the bridge.
The formula is not the whole legal answer
A calculated bridge-formula value does not override separate single-axle, tandem, gross, tire, equipment, state, route or permit restrictions. Treat it as one arithmetic check among several.
Measure the correct axle group
The result depends on which group you are evaluating and the distance between its outer axles. Using the wrong axle count or spacing produces a meaningless result even if the arithmetic is correct.
Use official route guidance
For compliance decisions, confirm the current federal and state rules and any route- or permit-specific requirements. TowAtlas performs the formula on the numbers you enter; it does not certify a load as legal.
Common questions
What does L mean in the bridge formula?
L is the distance in feet between the extreme axles of the axle group being evaluated.
Does passing the bridge formula mean the truck is legal?
Not necessarily. Other axle, tandem, gross, tire, equipment, state, route and permit limits can still apply.
Open the related TowAtlas calculator and use your saved Garage setup when available.