Bridge Formula Calculator

How the limits are calculated

The bridge formula gives the maximum weight on any group of two or more consecutive axles:

W = 500 × ( LN ÷ (N − 1) + 12N + 36 )

W is the weight in pounds, rounded to the nearest 500 lb, L is the distance in feet between the outer axles of the group, and N is the number of axles in the group. The calculator applies the formula to every group of consecutive axles, not only to the vehicle as a whole, and holds each group to no more than the gross vehicle weight limit of 80,000 lb.

Each axle is also held to the single axle limit of 20,000 lb, and each tandem axle to 34,000 lb. Two consecutive sets of tandem axles may carry 68,000 lb where the first and last axles of the two sets are 36 ft or more apart, even where the formula gives less.

Measuring axle spacing

Axle spacing is measured from the center of one axle to the center of the next. The calculator rounds the distance between the outer axles of each group down to whole feet, which gives the lower and therefore stricter limit, and rounds a value exactly halfway between two steps of 500 lb down, as the Federal Highway Administration's table does.

Axles whose centers are within 96 in of the first axle in a run are treated as one tandem axle (23 CFR 658.5). Axles spread further apart are each held to the single axle limit, and the group to the bridge formula.

Scope

The calculator applies the federal limits on the Interstate System. Limits on other roads are set by each State, and a State may authorize heavier vehicles by permit or under provisions this calculator does not apply. The guide Federal Bridge Gross Weight Formula sets out the formula and its exceptions in full.