What inputs does the Federal Bridge Formula use?

It uses the number of axles in the group and the distance in feet between the extreme axles in that group.

Does the bridge-formula result by itself determine legal weight?

No. Other federal, state, route, permit, axle, tire and equipment limits may also apply.

Federal Bridge Formula Calculator

Enter the distance in feet between the extreme axles in the group and the number of axles in that group.

Distance between extreme axles, L (ft)

Number of axles, N

Formula only

TowAtlas runs the arithmetic from the federal bridge formula. It does not determine route legality, state exceptions, permits or individual axle-group compliance.

Formula reference

Federal Bridge Formula arithmetic

The federal bridge formula is W = 500[(LN / (N - 1)) + 12N + 36], where W is the calculated weight in pounds, L is the distance in feet between the extreme axles in the group, and N is the number of axles in that group.

For example, entering 51 ft and 5 axles produces an unrounded formula result of 79,875 lb before the calculator applies its display rounding. That number is only one part of a legal-weight check.

What the result does not determine

Route, jurisdiction, individual axle and tandem limits, permits, exemptions, tire ratings and equipment ratings can impose additional restrictions. Treat this as formula arithmetic rather than a legal determination. Compare actual axle groups with the semi-truck axle calculator and the full scale-ticket total with the gross-weight calculator.

Keep this result in your TowAtlas history.