Federal Bridge Formula Calculator
Check a truck against the Federal Bridge Formula: allowed weight for every axle group, tandem and single-axle limits and the 80,000 lb gross cap.
The Federal Bridge Formula limits how much weight a truck may put on any group of two or more consecutive axles on the US Interstate system, so heavy loads are spread over enough length to protect bridges. Enter your axle spacings, and optionally the axle weights from a scale ticket, and the calculator checks every axle group against the formula, the 20,000 lb single-axle and 34,000 lb tandem limits and the 80,000 lb gross limit.
How to use this tool
- Measure the distance between the centres of each pair of neighbouring axles, front to back.
- Enter the spacings in feet, separated by commas.
- Optionally enter the weight on each axle to check for violations.
- Read the outer-group ceiling and every internal group; the ceiling alone does not establish legal loading.
Formula
W = 500 × (L × N ÷ (N − 1) + 12 × N + 36)
W is the maximum weight in pounds on a group of axles, 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 it. Axles more than 40 in and no more than 96 in apart form a tandem, limited to 34,000 lb. Two consecutive tandems may carry 34,000 lb each when their first and last axles are at least 36 ft apart.
Example
A five-axle tractor-semitrailer with spacings of 12, 4.25, 31 and 4 ft has 51.25 ft between its first and last axles, which rounds to 51 ft. W = 500 × (51 × 5 ÷ 4 + 60 + 36) = 79,875, rounded to 80,000 lb, the same value as FHWA’s Bridge Table. The tandems are 39 ft apart, allowing 68,000 lb on axles 2–5, so axle weights of 12,000 + 4 × 17,000 = 80,000 lb pass every check.
Frequently asked questions
Why does my truck fail when it is under 80,000 lb?
A short group inside the truck can be overloaded. FHWA’s own example is a four-axle truck at 57,000 lb that passes overall but carries 45,000 lb on three axles spread over 9 ft, where the formula allows only 42,500 lb.
How are fractions of a foot handled?
The Bridge Table is published in whole feet, so by default the tool rounds each distance to the nearest foot, with a half foot rounding up as a planning convention. The 36 ft exception always uses actual distance. Untick to use exact L; confirm enforcement rounding with the state.
Do state limits differ?
Yes. Some states keep grandfathered limits on the Interstate or issue permits, and states set their own rules on other roads. Check the state where you will travel.
Sources
Last updated: 2026