Federal Weight Limits and the Bridge Formula
Three numbers everyone knows, and a fourth rule that quietly overrides them. A legal-weight truck can still be over on an axle group, and the formula that decides it takes about a minute to learn.
The three federal limits
On the Interstate System, 23 CFR 658.17 sets:
- 80,000 pounds maximum gross vehicle weight, except where a lower gross weight is dictated by the bridge formula;
- 20,000 pounds on any one axle, including any one axle of a group of axles;
- 34,000 pounds on tandem axles.
Note the qualifier buried in the first one. Eighty thousand is a ceiling, not an entitlement. The formula can put you below it.
These are Interstate System limits. Weight on non-Interstate highways is governed by state law, and state limits are frequently different — sometimes higher, sometimes lower, and often with their own permit structures. A route that leaves the Interstate is a route where you need to know the state's rules.
The bridge formula
The Federal Bridge Gross Weight Formula exists because weight concentrated over a short distance stresses a bridge far more than the same weight spread out. It limits the weight on any group of two or more consecutive axles based on how far apart the outer axles of that group are.
W = 500 × [ LN ÷ (N − 1) + 12N + 36 ]
- W is the maximum overall gross weight on the group, 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.
You apply it to every possible group of consecutive axles on the vehicle, not just the obvious ones. A five-axle tractor-trailer has several groups to check: the drive tandem, the trailer tandem, the drives plus the trailer axles, the steer plus the drives, and the whole vehicle.
Worked through
Take the drive tandem on its own: two axles, 4 feet apart. W = 500 × [ (4 × 2) ÷ 1 + 24 + 36 ] = 500 × 68 = 34,000 pounds. Which is where the tandem limit comes from.
Now take a five-axle combination measuring 41 feet from the steer axle to the rear trailer axle. W = 500 × [ (41 × 5) ÷ 4 + (12 × 5) + 36 ] = 500 × [ 51.25 + 60 + 36 ] = 500 × 147.25 = 73,625 pounds. That vehicle cannot legally gross 80,000 on the Interstate no matter how carefully the axles are balanced, because the axles are not spread far enough apart.
Stretch the same five axles out to 51 feet and the formula gives 500 × [ 63.75 + 60 + 36 ] = 79,875 pounds, which the statutory 80,000 ceiling then governs. That is why the standard long-wheelbase tractor with a 53-foot trailer works at full weight and a short-coupled combination does not.
The 36-foot exception
There is a specific statutory exception: two consecutive sets of tandem axles may carry a gross load of 34,000 pounds each if the overall distance between the first and last axles of these tandems is 36 feet or more. That is the provision that makes the ordinary five-axle tractor-trailer work, and it is written into the law precisely because the formula alone would not permit it.
Grandfather rights
This is where the tidy picture ends. The federal limits do not apply to single-axle weights, tandem-axle weights or gross weights that were legally authorised under state law on July 1, 1956. There is a second preservation for vehicle combinations that met specifications on January 4, 1975, which keep those weights even if the state later adopts lower limits.
The practical result is a patchwork. Several states run Interstate segments at gross weights above 80,000 pounds under grandfather rights, and the configurations that qualify are defined state by state. Anyone telling you the national limit is 80,000 everywhere is giving you a useful approximation, not a fact you can plan a heavy load around.
Why your axle weights, not your gross, are what get you
A truck can be at 78,000 pounds gross — comfortably legal — and be 2,000 pounds over on the drives because the trailer tandem is in the wrong hole. Gross weight is the number on the ticket; axle distribution is the number that gets weighed.
Two adjustments control it on a standard combination:
- Sliding the trailer tandem moves weight between the drive axles and the trailer axles. Sliding it rearward moves weight onto the drives; forward moves it onto the trailer.
- Sliding the fifth wheel shifts weight between the steer axle and the drives.
How much weight moves per hole is specific to your equipment. The only way to know yours is to weigh, slide, weigh again, and write it down. Every experienced driver has that number for their own truck; it is not something anyone can tell you from a website.
Note also that the tandem slide affects your effective wheelbase, and therefore the bridge formula. Sliding the trailer tandem all the way forward to fix an axle weight can put you out of compliance on a bridge group.
Overweight permits
Everything above describes legal weight. Above it you are into permitted movement, and permits are issued by each state for travel on its own roads. There is no federal overweight permit for the Interstate System.
That means a multi-state overweight run is a separate permit from each state, each with its own fee, its own route restrictions, its own hours of travel and its own escort requirements. Permit services exist because assembling that for a single load is genuinely a job. If you are doing it occasionally, use one; if you are building a business on it, learn it.
Related reading
- Cargo securement — the other thing a scale house cares about.
- Flatbed freight — where weight and securement decide the job.
- Out-of-service orders — what an overweight or unsecured load can trigger.
Sources
- 23 CFR 658.17 — Weight, for the federal limits and grandfather provisions.
- Bridge Formula Weights — Federal Highway Administration, for the formula and the 36-foot exception.
General information, not legal advice. Regulations change; confirm current requirements with the agency before acting.