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Haul Math

Before you take the load

Bridge Formula Calculator

Check every group of axles on your rig against the federal bridge formula, plus the single, tandem, and gross limits on the Interstate.

Axle spacings

Center of one axle to the center of the next, in feet. Axle 1 is the steer axle. 4 ft 6 in = 4.5.

Axle weights (optional)

From your scale ticket. Leave all blank to see the limits only.

Your result

Enter your axle spacings and tap "Check bridge formula". A 5-axle tractor-trailer is filled in.

How this calculator works

W = 500 × (L × N ÷ (N − 1) + 12 × N + 36)

W is the most any group of two or more consecutive axles may carry, rounded to the nearest 500 lb. L is the distance in feet between the first and last axle of the group, and N is the number of axles in the group. The calculator checks every group: the whole truck (the "outer bridge"), the tractor, the trailer, and everything in between.

  • Any one axle: 20,000 lb. Axles 40 inches or less apart count as one axle.
  • Tandem (axles more than 40 and not more than 96 inches apart): 34,000 lb. This replaces the formula for those axles.
  • Gross: 80,000 lb on the Interstate, or less if the formula says so.
  • Two consecutive tandems may carry 34,000 lb each if the first and last axles of the two tandems are 36 feet or more apart, even where the formula gives a little less.

Rounding. FHWA's bridge table rounds to the nearest 500 lb and rounds down when the result lands exactly halfway. It reads spacings in whole feet, using the 8-foot row for anything more than 8 but less than 9 feet, so the calculator drops any fraction of a foot. That never gives you more weight than the table.

These are the federal limits for the Interstate System. Some states have higher grandfathered limits, and states set their own limits on other roads. See truck weight limits by state. Official guidance: FHWA Bridge Formula Weights.