Why can a truck be overweight on axles but under 80,000 lb total?

Because the bridge formula and the gross vehicle weight cap are two separate checks. A rig can total less than 80,000 lb and still fail if one axle group — commonly the drive tandem or the span between the drive and trailer tandems — carries more weight than its spacing allows under the formula.

How do I fix a bridge formula violation?

The fastest fix is usually sliding the trailer's tandem axles further back, which increases the spacing between axle groups and raises the legal maximum for the affected group. Redistributing cargo to shift weight off the overloaded group is the other common fix when the tandem is already at its rearmost slide position.

What are the fixed axle weight limits regardless of spacing?

Federally, a single axle is capped at 20,000 lb and a tandem (2-axle) group at 34,000 lb, no matter how the bridge formula spacing works out. The formula can only reduce the legal maximum below these caps at tighter spacings — it never raises a group above them.

Does the Bridge Formula apply to single axles?

No — the formula itself governs groups of two or more consecutive axles. A single, isolated axle is checked only against the flat 20,000 lb federal cap, not the bridge equation.

Do state limits differ from the federal Bridge Formula?

Some states allow modified formulas or higher caps under specific permits, particularly for certain configurations or non-interstate roads. The federal formula and caps in this calculator apply to interstate highways; always confirm current limits with the relevant state DOT for a specific route or permit.

Bridge Formula Calculator — Federal Axle Weight & Sliding Tandem Check

What Is the Federal Bridge Formula — and Why Sliding Your Tandem Matters

The Federal Bridge Gross Weight Formula (23 CFR 658.17) sets the maximum weight a group of axles can carry based on how far apart those axles are spread — not just how many there are. It exists to keep concentrated axle loads from damaging bridges on the U.S. interstate system, and it applies on top of the 80,000 lb gross vehicle weight cap, not instead of it.

In practice, this is why a legally-loaded truck can still get written up at the scale: the total weight is under 80,000 lb, but one specific axle group — most often the drive tandem or the gap between the drive and trailer tandems — is carrying more than its spacing allows. Sliding the trailer tandem back a few feet is usually enough to fix it, because it increases the spacing the formula uses for that group.

What drives your legal axle weight

Axle spacing

The distance between the outer axles of a group is the single biggest lever — more spread means a higher legal weight for that group.

Axle count

More axles in a group spread the load further, but each additional axle also raises the group's legal weight less than the one before it.

Sliding tandems

Most 53ft trailers let the rear tandem slide fore and aft. Sliding it back increases the drive-to-trailer-tandem spacing, which is the fastest fix for a bridge violation at the scale.

Fixed axle caps

Regardless of spacing, a single axle is capped at 20,000 lb and a tandem at 34,000 lb federally — the bridge formula can only lower that ceiling, never raise it.

The Bridge Formula equation

W = 500 × ( (L×N)/(N−1) + 12×N + 36 )

W = max weight (lb) · L = span between outer axles (ft) · N = number of axles in the group

The result is rounded down to the nearest 500 lb. A single axle is always capped at 20,000 lb, and a tandem (2-axle) group is capped at 34,000 lb even if the formula would allow more — the fixed caps and the formula both apply, and the lower of the two governs.

What happens if an axle group is over the limit?

A bridge formula violation is cited separately from an overweight gross vehicle citation, and fines apply per axle group, sometimes stacking if more than one group is out of compliance. Because the fix is usually just repositioning the trailer's sliding tandem — no reload required — it is worth checking before you leave the yard, not after the scale flags it. Exact fines and enforcement tolerances vary by state, so always confirm with the relevant DOT for shipments near the limit.

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Bridge Formula Calculator