Federal Bridge Formula and Axle Weight Basics
An introduction to the Federal Bridge Gross Weight Formula and how axle spacing — not just total weight — determines whether a load is legal.
Written for: Fleet managers, dispatchers. This guide is general educational information — permit, escort, and compliance requirements vary by state and by load. Contact our team to confirm what applies to your specific move.
Why axle spacing matters as much as total weight
Federal law limits gross vehicle weight to 80,000 lbs on the Interstate system, but total weight alone doesn't determine legality. The Federal Bridge Gross Weight Formula (23 CFR 658.17) sets a maximum weight for any group of axles based on the number of axles in the group and the distance between the outermost axles in that group — spreading weight over a longer wheelbase allows more weight to be carried legally.
The formula
The formula is expressed as W = 500 [ LN / (N − 1) + 12N + 36 ], where W is the maximum weight in pounds carried by any group of N or more axles, and L is the spacing in feet between the outermost axles of that group. In practice, carriers rarely calculate this by hand for every configuration — it's applied through axle weight tables or software once a trailer configuration and axle spacing are known.
Single, tandem, and tridem axle limits
On top of the bridge formula, most states also cap individual axle groupings — commonly around 20,000 lbs for a single axle, 34,000 lbs for a tandem axle group, and higher, state-specific limits for tridem groups. Overweight permits typically allow these per-axle limits to be exceeded, but only up to the state's permitted maximum, and only when the load is properly distributed across the axle groups.
Why this matters before dispatch
Understanding axle weights before a load is dispatched — not after it's already loaded — avoids the two most common outcomes of a bad axle configuration: a citation at the scale, or a trailer swap that delays the whole move.