Contents
- The formula
- Example 1: drive tandem group
- Example 2: five-axle combination, axle spacing determines the gross ceiling
- Example 3: axle group over while gross is under
- State and permit checks
- Sources
The formula
The federal bridge formula from FHWA is:
W = 500 × ( LN / (N − 1) + 12N + 36 )
- W = maximum allowable weight in pounds for the axle group
- L = distance in feet between the outer axles of the group
- N = number of axles in the group
The examples below apply this formula with real numbers. Use the FHWA source linked here to confirm current federal wording before relying on any result.
Example 1: drive tandem group
A standard drive tandem has two axles (N = 2) spaced 4 feet apart (L = 4).
W = 500 × ( (4 × 2) / (2 − 1) + 12 × 2 + 36 ) W = 500 × ( 8 + 24 + 36 ) W = 500 × 68 W = 34,000 lb
This is where the familiar 34,000 lb tandem limit comes from. The spacing and axle count together produce that ceiling, not an arbitrary round number.
Example 2: five-axle combination, axle spacing determines the gross ceiling
A standard five-axle tractor-trailer has N = 5 axles. The bridge formula is applied to the full group from the steer axle to the last trailer axle.
At L = 48 ft between outer axles:
W = 500 × ( (48 × 5) / (5 − 1) + 12 × 5 + 36 ) W = 500 × ( 60 + 60 + 36 ) W = 500 × 156 W = 78,000 lb
The bridge formula caps this combination at 78,000 lb even though the general gross limit is 80,000 lb. The axle spacing is too short.
At L = 51 ft between outer axles:
W = 500 × ( (51 × 5) / (5 − 1) + 12 × 5 + 36 ) W = 500 × ( 63.75 + 60 + 36 ) W = 500 × 159.75 W = 79,875 lb
Now the combination fits under 80,000 lb gross and clears the bridge formula at this spacing. Sliding trailer tandems to change L can shift this result in either direction, which is why re-weighing after a tandem move matters.
Example 3: axle group over while gross is under
A truck weighs 78,500 lb gross — comfortably under 80,000 lb. The scale ticket shows:
- Steer: 12,000 lb
- Drive tandem: 35,200 lb
- Trailer tandem: 31,300 lb
The drive tandem at 35,200 lb exceeds the 34,000 lb tandem limit from Example 1. The gross number does not reveal this problem. That is why a scale ticket must be read by each axle group, not only by total weight.
State and permit checks
These examples use the federal baseline for Interstate operation. State rules and permit terms can be more or less restrictive depending on route, road type, and load category. After applying the federal formula, check the relevant state page for state-specific limits before moving.
Sources
FHWA Bridge Formula Weights is the controlling federal source for the formula used in this explanation.
FAQ
Where does the 34,000 lb tandem limit come from?
It follows directly from the FHWA bridge formula applied to a standard two-axle tandem group with 4 ft spacing. Example 1 on this page shows the calculation.
Why can a five-axle truck be over the bridge formula even under 80,000 lb gross?
The bridge formula depends on axle spacing, not just total weight. A shorter wheelbase produces a lower allowable weight. Example 2 shows two spacing scenarios.
Should I check state sources too?
Yes. The examples use federal Interstate baselines. State rules, route postings, and permit terms can set different limits on non-Interstate roads.