Illinois truck weight limits
On Interstate highways in Illinois, federal weight limits apply. Four checks decide whether your truck is legal — and the one that surprises most drivers is the Federal Bridge Formula, which can fail a truck that passes every other limit.
Federal limits on Illinois Interstates
| Limit | Maximum | Applies to |
|---|---|---|
| Single axle | 20,000 lb | Any one axle |
| Tandem axle group | 34,000 lb | Axles 40–96 inches apart |
| Tridem axle group | governed by the bridge formula | 3 axles in a group (no flat federal cap) |
| Gross vehicle weight | 80,000 lb | The whole rig |
| Bridge formula | varies by spacing | Every group of 2+ axles |
The bridge formula check
L is the spacing in feet between the outer axles of a group, N the number of axles. A standard 5-axle rig at 51 ft is capped at 79,500 lb by this formula — under the 80,000 lb gross limit. Full explanation with worked examples.
Illinois-specific rules
- Statewide limits match federal on most roads (625 ILCS 5/15-111): 20,000 lb single, 34,000 lb tandem, 80,000 lb gross; tridem groups up to 42,500 lb.
- Local roads and structures can be posted lower than the statewide limits — the posting controls.
- Oversize/overweight permits are issued by IDOT; see their legal weights & dimensions tables for axle-spacing specifics.
US-IL and the verdict applies Illinois-specific limits on top of the federal baseline.Illinois truck weight — frequently asked questions
How much can a truck weigh in Illinois?
On Illinois Interstates the federal gross-weight limit is 80,000 lb, with a 20,000 lb single-axle limit, a 34,000 lb tandem limit and the Federal Bridge Formula. Illinois sets its own limits on non-Interstate roads — see the Illinois-specific rules on this page.
Does Illinois follow federal truck weight limits?
On the Interstate system, yes — federal limits govern in every state. Off the Interstate, Illinois applies its own axle, gross and permit rules, detailed on this page where they have been verified against the official state source.
What is the single-axle weight limit in Illinois?
The federal single-axle limit of 20,000 lb applies on Illinois Interstates. Some states set a different single-axle figure on non-Interstate roads — check the Illinois-specific section above for the governing number.
What is the maximum tandem-axle weight in Illinois?
On Illinois Interstates the federal tandem limit is 34,000 lb. Several states allow a different tandem weight on non-Interstate roads — see the Illinois exceptions above for the figure that governs.
Scaled heavy in Illinois?
If the trailer tandems are over, the fix is usually a computed tandem slide, not unloading — see how sliding tandems shifts weight. And here's what an overweight ticket really costs before you decide to run it.
Run your scale-ticket weights through the federal and bridge-formula calculator in your browser. The full platform predicts axle loads from load placement before you load, computes the exact slide-to-legal fix, and applies Illinois rules on top of the federal baseline.
Open the free calculatorOur calculator runs the federal single, tandem, gross and Bridge Formula checks in seconds. The full platform adds per-state limits, axle-load prediction and the slide-to-legal fix.
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