Federal Bridge Formula Calculator
Calculate the maximum legal gross weight for a group of axles under the Federal Bridge Gross Weight Formula (49 CFR 658.17).
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Enter axle count and spacing, then Calculate.
How this works
The Federal Bridge Formula limits how much weight can rest on a group of consecutive axles based on how many axles are in the group and how far apart the outermost two are — spreading weight over more, wider-spaced axles is what allows heavier gross weights without exceeding the formula.
W = 500 × ( (L × N) / (N − 1) + 12N + 36 )
W = max gross weight (lbs, rounded down to the nearest 500) · L = feet between the first and last axle in the group · N = number of axles in the group. A two-axle group spaced under 8ft is capped at 34,000 lbs regardless of the formula result. Federal single-axle max is 20,000 lbs; tandem-axle max is 34,000 lbs.
This result is the axle-group limit only. The flat federal 80,000 lb gross vehicle weight cap (and any applicable state overweight-permit thresholds) apply separately and independently — a vehicle must satisfy both the Bridge Formula result for every axle group and the overall GVW cap; whichever is lower governs.
Reference tool only — some states apply their own bridge formula variant for intrastate travel, and a real permit requires a full axle-configuration review. Always verify with the state before you travel. See Permit Requirements by State.