The federal bridge formula, explained for drivers

What it is, the numbers that matter on a 5-axle rig, and how sliding changes it

In short: the bridge formula limits how much weight a group of axles can carry based on how many axles are in the group and how far apart the outer axles are. More axles or more distance means more allowed weight. Whichever limit is lowest (axle, tandem, gross or bridge) is the one that applies (FHWA).

The formula

W = 500 × ( LN ÷ (N − 1) + 12N + 36 )

W is the maximum gross weight on the axle group, to the nearest 500 lb.
L is the distance in feet between the outer axles of the group.
N is the number of axles in the group.

That's the formula as FHWA publishes it (FHWA Bridge Formula Weights).

The limits that sit on top of it

Single axle: 20,000 lb (axles 40 in or less apart count as one).
Tandem axle: 34,000 lb for axles more than 40 in and not more than 96 in apart.
Gross: 80,000 lb on the Interstate System, unless the formula sets a lower number.

Source: FHWA.

The 36-foot exception

Two consecutive sets of tandems can carry 34,000 lb each if the distance between the first and last axles of those tandems is 36 ft or more. On a 5-axle tractor-trailer, that's axle 2 (first drive) to axle 5 (last trailer axle). Without this exception, the formula would only allow 66,000 to 67,500 lb on tandems spaced 36 to 38 ft apart (FHWA).

Quick table for common spacings

Calculated from the formula above, rounded to the nearest 500 lb. The 5-axle column is capped at 80,000 lb.

Outer axle distance (L)	4 axles (axles 2–5)	5 axles (axles 1–5)
40 ft	68,500 lb	73,000 lb
44 ft	71,500 lb	75,500 lb
46 ft	72,500 lb	77,000 lb
48 ft	74,000 lb	78,000 lb
50 ft	75,500 lb	79,000 lb
51 ft	76,000 lb	80,000 lb
52 ft	76,500 lb	80,000 lb
54 ft	78,000 lb	80,000 lb
57 ft	80,000 lb	80,000 lb

FHWA's own example: a 5-axle rig with 51 ft between axles 1 and 5 is allowed the full 80,000 lb (FHWA). Check your exact numbers against the official bridge table before relying on any one figure.

What it means when you slide

Sliding the trailer tandems forward shortens the distance between your drives and trailer tandems. Slide far enough forward and you can lose the 36 ft exception or the 80,000 lb gross. Sliding back lengthens it, but can put you over a state's kingpin-to-rear-axle limit. Axle Balance Pro checks both when it suggests a hole.

Know which way to slide before you get back on the scale. Axle Balance is a free calculator made by a trucker. Enter your scale ticket and it tells you which way to slide your tandems and how many holes, plus your fifth wheel. Works offline in the cab.

Open the free calculator

Estimates and general information only, not legal advice. Laws change and enforcement varies. Always confirm on a certified scale and with the state before you rely on it. See the Terms of Use.