Bridge Formula Weights

Contents

• The formula
• Common configuration thresholds
• Practical reading order
• What this reference does not replace
• Sources

The federal bridge formula sets a maximum weight for each axle group based on the number of axles in the group and the distance between the outer axles. It is the rule that explains why 80,000 lb gross is not automatically safe for every vehicle configuration.

This page is a quick reference for the weight thresholds that the formula produces for common truck configurations. For a full driver-focused explanation of how the formula works, see Bridge Formula Explained. For worked numerical examples, see Bridge Formula Calculation Examples.

The formula

W = 500 × ( LN / (N − 1) + 12N + 36 )

• W — maximum allowable weight in pounds for the group
• L — distance in feet between the outer axles of the group
• N — number of axles in the group

Common configuration thresholds

These are the results the formula produces for standard Interstate configurations. They are starting points — state rules, permits, and equipment limits can differ.

Two-axle tandem (N=2, L=4 ft) Formula result: 34,000 lb. This is the source of the familiar tandem axle limit. Both the drive tandem and the trailer tandem on a standard five-axle combination are measured against this number.

Single axle (N=1) The federal single-axle limit is 20,000 lb. The bridge formula does not add a separate limit for N=1; the 20,000 lb figure comes from the statutory axle limit directly.

Five-axle combination at 48 ft outer spacing (N=5, L=48) Formula result: 78,000 lb. This is below the 80,000 lb gross ceiling. A five-axle combination with short overall spacing cannot reach 80,000 lb legally under the bridge formula even if every individual axle group is within its own limit.

Five-axle combination at 51 ft outer spacing (N=5, L=51) Formula result: 79,875 lb. At this spacing the combination fits under 80,000 lb gross and clears the bridge formula. Sliding trailer tandems to lengthen the outer spacing can shift this result.

Practical reading order

Compare each axle group from the scale ticket in this order:

1. Drive tandem — most likely to be near a limit on a loaded combination
2. Trailer tandem — check whether a tandem slide changes the result
3. Steer axle — compare to the single-axle limit and equipment ratings
4. Gross weight — compare to the 80,000 lb ceiling
5. Bridge formula — apply to the full combination when spacing is close to a threshold

What this reference does not replace

This page summarizes federal Interstate thresholds. State rules and non-Interstate routes can produce different limits. For an actual movement, open the FHWA source and the applicable state page before relying on any number from this reference.

Sources

FHWA Bridge Formula Weights and FHWA Federal Size and Weight Regulations are the controlling federal sources.