Free practice tests for all 50 states. The only site with a real Federal Bridge Formula Calculator, printable flashcards, and interactive study guides — all in one place.
Enter your axle configuration to instantly calculate your maximum legal gross weight under the Federal Bridge Formula (Formula B). Used by truck drivers, CDL students, and DOT officials nationwide.
```| Axle Type | Federal Limit | Notes |
|---|---|---|
| Single Axle | 20,000 lbs | Axles ≤ 40" apart |
| Tandem Axle | 34,000 lbs | 40"–96" spacing |
| Gross Vehicle Weight | 80,000 lbs | Interstate max |
| 5-Axle Exception | 80,000 lbs | Tandems ≥36 ft apart |
| APU/Idle Reduction | +550 lbs | With certification |
Enacted by Congress (23 U.S.C. 127) to protect bridges from concentrated axle loads. The formula limits the weight-to-length ratio of commercial vehicles.
The CDL General Knowledge test frequently asks about weight limits. The three numbers to memorize: 20,000 (single axle), 34,000 (tandem axle), 80,000 (gross weight). The bridge formula governs all axle combinations beyond tandem.
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