Trucking · U.S. weight limits
Federal bridge formula calculator.
Enter the distance between the outer axles of a group and how many axles are in it to get the maximum gross weight for that group.
Axle group
Result
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How it works
W = 500 × (L × N ÷ (N − 1) + 12 × N + 36), where W is the overall gross weight in pounds on any group of two or more consecutive axles, 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 (FHWA). In no case may the total gross weight exceed 80,000 lb (23 CFR 658.17(e)). When the raw result falls exactly halfway between two 500 lb steps, this tool rounds down to match FHWA’s published example. Worked examples from FHWA: L = 20 ft and N = 3 gives 51,000 lb; L = 51 ft and N = 5 gives 80,000 lb; L = 35 ft and N = 4 gives 65,500 lb; L = 9 ft and N = 3 gives 42,500 lb.
What this does not decide
The Bridge Formula is one of three federal checks, alongside the 20,000 lb single-axle and 34,000 lb tandem limits and the 80,000 lb gross limit. FHWA notes that two consecutive tandem sets may each carry 34,000 lb when the first and last axle are at least 36 feet apart. States may allow more under grandfather rights and may issue permits for nondivisible loads. FHWA notes that the statute and regulation control in any dispute. This is general information, not legal or compliance advice.
Sources (each retrieved 2026-10-16): FHWA, Bridge Formula Weights (formula, definitions of W, L and N, nearest 500 lb, worked examples, 36 ft tandem exception); 23 CFR 658.17 Weight (20,000 lb single axle, 34,000 lb tandem, 80,000 lb gross, bridge gross weight formula).
FAQ
What is the federal bridge formula?
It is W = 500(LN/(N-1) + 12N + 36), which limits the weight on a group of consecutive axles based on how far apart the outer axles are and how many axles are in the group. It was set up to protect bridges.
What does L mean in the bridge formula?
L is the distance in feet between the outer axles of the group you are checking, and N is the number of axles in that group.
What is the maximum gross weight under the bridge formula?
Federal rules cap the gross vehicle weight at 80,000 lb on the Interstate system, even where the formula would allow more.
Does a 5-axle truck meet the bridge formula at 80,000 lb?
Only if the outer axles are far enough apart. FHWA shows 51 ft between the first and last axle of a five-axle group allows exactly 80,000 lb.
Does this replace the state weight limits?
No. States can set higher grandfathered limits or require permits, and the weight on each group must also meet axle limits. Check the rules where you operate.