What Is Federal Bridge Formula in Trucking?
Also called: Bridge Gross Weight Formula, Bridge Formula Weights, Axle Spacing Formula
The federal bridge formula limits how much weight a group of axles may carry, based on how many axles there are and how far apart they sit. It applies on the Interstate. Congress set it in 1975 to protect bridges. It works with the 20,000-pound single axle, 34,000-pound tandem axle and 80,000-pound gross weight limits.
Last reviewed September 23, 2026 · Facts checked October 21, 2026
Key Points
- The formula is W = 500 x (LN / (N - 1) + 12N + 36). W is the most weight in pounds for a group of two or more consecutive axles, to the nearest 500 pounds
- L is the distance in feet between the outer axles of the group. N is the number of axles in the group (23 CFR 658.5)
- The federal limits are 20,000 pounds on a single axle, 34,000 pounds on a tandem axle and 80,000 pounds gross, unless the bridge formula allows less (23 CFR 658.17)
- Two consecutive sets of tandem axles may each carry 34,000 pounds if the distance between the first and last axle is 36 feet or more (23 CFR 658.17(e))
- The rule covers the Interstate and reasonable access to it (23 CFR 658.17(a)). On the Interstate, a State may not enforce lower limits than these (658.17(f))
- The weights include all tolerances, except a scale allowance when using portable scales that may not be more than 5 percent (658.17(g))
- To fix a bridge formula violation, remove load or increase the spacing between axles (FHWA)
What is the federal bridge formula?
It is a federal rule that ties the weight a truck may carry to the number of its axles and the distance between them. Congress enacted it in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. A truck can spread weight over more axles, or put more distance between axles, to carry more. FHWA says the formula sets the most weight any set of axles may carry on the Interstate.
How do you read the bridge formula?
W is the most weight, in pounds, for a group of two or more consecutive axles, rounded to the nearest 500. L is the distance in feet between the outer axles of the group. N is the number of axles in the group. The formula is W = 500 x (LN / (N - 1) + 12N + 36). Take five axles spread over 51 feet. LN is 255, divided by 4 is 63.75. Add 60 and 36 to get 159.75. Times 500 is 79,875, which rounds to 80,000 pounds. In FHWA's example, four axles over 35 feet may carry 65,500 pounds, so 68,000 pounds on them is a violation.
How does the bridge formula work with the 80,000-pound limit?
You check several limits. The gross weight may not pass 80,000 pounds, unless the bridge formula allows less. One axle may carry 20,000 pounds. A tandem axle may carry 34,000 pounds. Then any group of two or more consecutive axles must also pass the bridge formula, even if the single, tandem and gross weights are fine. FHWA says the axle groups to check on a common five-axle tractor-trailer are axles 1 to 3, 1 to 5 and 2 to 5.
Where does the bridge formula apply?
The federal rule applies to the Interstate and reasonable access to it (23 CFR 658.17(a)). A State may not enforce lower single, tandem or bridge formula weights on the Interstate, up to 80,000 pounds, including enforcement tolerances (658.17(f)). Off the Interstate, State law decides, so check the State you are in.
Are there exceptions to the bridge formula?
Yes. States may issue special permits for loads that cannot be divided, without regard to the axle, gross or bridge formula limits (23 CFR 658.17(h)). Weights that were legal under State law on certain older dates are grandfathered (658.17(i)). A vehicle with an auxiliary power unit that cuts idling may carry up to 400 more pounds if the unit's weight is certified and it works (658.17(n)).
Related Free Tools
Low Bridge Lookup
See known low bridges and overhead clearances near a place or in a state, checked against your truck's height. From the federal National Bridge Inventory.
Weight & Axle Load Calculator
Check federal axle weight limits and bridge formula compliance before you scale.
Scale / Weigh Station Guide
PrePass eligibility, what to do at a scale, violations, and a state-by-state guide.
Oversize / Overweight Permit Estimator
Check whether your load goes past the usual legal height, width, trailer length and weight, and when escorts often start.
Frequently Asked Questions
What is the bridge formula in trucking?+
A federal rule that limits the weight any group of two or more consecutive axles may carry on the Interstate, based on the number of axles and the distance between them (23 CFR 658.5 and 658.17).
What is the bridge formula equation?+
W = 500 x (LN / (N - 1) + 12N + 36). W is the most weight in pounds, to the nearest 500. L is the distance in feet between the outer axles. N is the number of axles (23 U.S.C. 127(a)).
What is the bridge formula for?+
Congress enacted it in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. A longer truck spreads its weight, which puts less stress on a bridge.
Does the bridge formula apply off the Interstate?+
The federal rule covers the Interstate and reasonable access to it (23 CFR 658.17(a)). State law sets limits on other roads, so check the State.
How do I fix a bridge formula violation?+
FHWA says to remove some load or increase the spacing between axles. A scale tool or your axle weights show which axle group is over.
What are the federal axle weight limits?+
20,000 pounds on a single axle, 34,000 pounds on a tandem axle and 80,000 pounds gross, unless the bridge formula allows less (23 CFR 658.17(b) to (d)).
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