Federal Bridge Formula calculator
Free Federal Bridge Formula calculator. Enter axle count and the spacing between the extreme axles to get the maximum legal weight (W = 500(LN/(N-1)+12N+36)).
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What the Bridge Formula does
The Federal Bridge Formula limits how much weight may rest on any group of consecutive axles based on how far apart they are spread. Weight concentrated over a short distance stresses bridges far more than the same weight spread out, so the formula rewards longer axle spacing with a higher legal limit.
The formula is:
W = 500 × ( L·N / (N − 1) + 12·N + 36 )
- W — maximum weight in pounds on the group (rounded down to the nearest 500 lb).
- L — distance in feet between the first and last axle of the group.
- N — number of axles in the group.
You must satisfy the Bridge Formula for every combination of consecutive axles, not just the outermost pair — and the single-axle (20,000 lb), tandem (34,000 lb), and gross (80,000 lb) caps still apply on top of it.
Worked example
A 5-axle tractor-semitrailer with 51 ft between the steer axle and the rear trailer axle: W = 500 × (51×5/4 + 12×5 + 36) = 500 × 159.75 = 79,875 → rounded down to 79,500 lb on that group. The 80,000 lb gross cap governs the whole vehicle.
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Frequently asked questions
What is the Federal Bridge Formula?
It is a federal rule (W = 500(LN/(N−1) + 12N + 36)) that sets the maximum weight allowed on any group of consecutive axles based on how many axles there are and how far apart the outer two are. Weight over a short span stresses bridges more, so the formula rewards longer axle spacing with a higher limit.
How do I calculate the bridge formula?
Take L (feet between the extreme axles of the group) and N (number of axles), then compute W = 500 × (L·N/(N−1) + 12N + 36) and round down to the nearest 500 lb. The calculator on this page does it for you — enter L and N.
What is the maximum weight on 5 axles?
It depends on the spacing. A typical 5-axle tractor-semitrailer with about 51 ft between the steer and rear trailer axle is allowed roughly 79,500 lb on that group by the bridge formula, and the 80,000 lb federal gross limit caps the whole vehicle.
Does the bridge formula apply on every road?
The Federal Bridge Formula governs the Interstate system nationwide. Many states apply it (or their own axle-spacing rules) on state highways too, and some states have grandfather or spacing-based limits that differ. Always check the specific state.
Is 80,000 lb the same as the bridge-formula limit?
No. 80,000 lb is the federal gross-weight cap for the whole vehicle. The bridge formula separately limits each group of axles by spacing — you must satisfy both, so a load can be under 80,000 lb and still illegal on one axle group.
Related
Axle weight limit checker (federal)Check steer, drive, and trailer axle weights plus gross weight against the US federal limits: 20,000 lb single, 34,000 lb tandem, 80,000 lb gross.What is the Federal Bridge Formula?The Federal Bridge Formula (W = 500(LN/(N-1)+12N+36)) sets the maximum legal weight on any group of axles based on how far apart they are spread.What is a tandem axle?A tandem axle is two axles spaced close together (typically 40–96 inches) that share a load as one group. The US federal tandem limit is 34,000 lb.Truck weight & length converterConvert pounds to kilograms, US tons to metric tonnes, feet to metres, and inches to centimetres — for cross-border (US/Canada) load planning.