Federal Bridge Formula & Axle Weight Legal Limit Calculator
Verify legal gross vehicle weight compliance using US DOT Federal Bridge Law B [W = 500(LN/(N-1) + 12N + 36)], inspect individual axle scale tolerances, and prevent weigh station citations.
Core Mathematical Formula: Federal Bridge Formula B: W = 500 × [ (L × N) / (N - 1) + 12N + 36 ]. W = Maximum permissible gross weight in lbs on any axle group; L = Distance in feet between the extreme axles of the group; N = Number of axles in the group. Statutory caps: Single Axle = 20,000 lbs; Tandem Axle = 34,000 lbs; Gross Combination = 80,000 lbs.
CAT Scale / Platform Scale Weights (lbs)
- • Steer Axle: 12,000 lbs (tire rating dependent)
- • Tandem Axles: 34,000 lbs (40"–96" spread)
- • Single Axle: 20,000 lbs
- • Gross Limit: 80,000 lbs cap without permit
- • Steer Axle: 5,500 kg (~12,125 lbs)
- • Tandem Axles: 17,000 kg (~37,478 lbs)
- • Tridem Axles: 21,000–24,000 kg
- • Super-B / B-Train: up to 63,500 kg gross
Federal Bridge Formula B Master Weight Table Excerpt [23 CFR § 658.17]
Official DOT StandardsThe table below indicates the maximum legal gross weight (in pounds) permitted on any group of two or more consecutive axles based on axle spacing (wheelbase in feet):
| Spacing (L in ft) | 2 Axles | 3 Axles | 4 Axles | 5 Axles | 6 Axles | 7 Axles |
|---|---|---|---|---|---|---|
| 8 ft | 34,000 | 34,000 | — | — | — | — |
| 12 ft | 35,000 | 43,000 | 47,000 | — | — | — |
| 20 ft | 39,000 | 47,500 | 52,000 | 56,500 | 61,000 | — |
| 28 ft | 40,000 | 52,500 | 57,500 | 62,500 | 67,500 | 72,500 |
| 36 ft (Inner Bridge) | — | 57,500 | 63,000 | 68,000 | 73,500 | 79,000 |
| 44 ft | — | 60,000 | 68,000 | 73,500 | 79,000 | 80,000 |
| 51+ ft (Outer Bridge) | — | — | 72,500 | 80,000 (CAP) | 80,000 | 80,000 |
Understanding Inner Bridge vs. Outer Bridge Compliance
Measures the total span from the steer axle to the rearmost trailer axle. On a standard 53-ft tractor-trailer, this distance is typically 51 to 55 feet, allowing the full legal maximum cap of 80,000 lbs gross vehicle weight.
Measures the span between the drive tandem and the trailer tandem (axles 2, 3, 4, 5). Under Bridge Formula B, an inner bridge span of 36 feet permits 68,000 lbs (34k drive + 34k trailer). If tandems are slid too close, inner bridge violations can occur even when gross is under 80k.
• Slide tandems BACK (toward rear): Puts more weight onto the tractor drive axles.
• Slide tandems FORWARD (toward front): Puts more weight onto the trailer tandem axles.
• Weight Transfer: Each pin hole shift typically transfers 250 to 400 lbs between axle groups.
In California (and states adhering to STAA strict rules), the distance from the kingpin to the center of the rear trailer axle must never exceed 40 feet. Sliding tandems too far back to legally carry trailer weight will trigger immediate CHP citations at California state weigh stations.
All calculations execute strictly according to standard federal transportation benchmarks and transparent mathematical constants with zero hidden padding.
A loaded tractor-trailer pulls onto a CAT scale with 51 feet outer wheelbase spacing (steer axle to rear trailer axle). The scale ticket reads Steer: 11,800 lbs, Drive Tandem: 34,500 lbs, Trailer Tandem: 33,100 lbs.
US DOT vs. Transport Canada Standards Comparison
US Interstate: Standard 5-axle semi is capped at 80,000 lbs (36,287 kg) gross, with 34,000 lbs tandem and 20,000 lbs single axle limits under Federal Bridge Formula B.
Canada (NSC/MOU): Standard tandem is permitted up to 17,000 kg (37,478 lbs). Canadian B-Train Super-B configurations gross up to 63,500 kg (140,000 lbs) under Ontario SPIF and Western Canadian regulations.
US FMCSA: Max 11 hours driving within a 14-hour on-duty window, followed by 10 consecutive hours off-duty/sleeper berth, and a 70-hour / 8-day rolling cycle.
Canada NSC Standard 9: Max 13 hours driving within a 14-hour on-duty window, requiring 10 hours daily off-duty, operating under Cycle 1 (70h/7d) or Cycle 2 (120h/14d).
US EIA Benchmark: Tied to the weekly US Energy Information Administration national on-highway diesel index, billed in $/mile.
Canadian FCA / NRCan: Freight Carriers Association (FCA) of Canada weekly diesel rack pricing, pegged in Canadian cents per liter (cpl) and billed in $/km or % surcharge.
Why Independent Truckers Choose TruckerToolsHub vs. Spreadsheets & Paid Apps
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Instant 1-click conversion between US Imperial (miles, gal, lbs) and Canadian Metric (km, liters, kg, FCA cpl) for seamless cross-border freight.
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Expert Regulations & Calculations Q&A
Authored by Commercial Freight Specialists • Mathematical logic verified against federal transportation law
Formulas aligned with standard US DOT FMCSA (49 CFR) and Bridge Formula B (23 CFR § 658.17).
Metric distances, liters, and MOU tandem axle limits verified with Transport Canada NSC Standard 9.
100% Client-Side calculation. Zero private load rates or carrier accounting data saved to any server.
Official In-Depth Industry Guides for Federal Bridge Formula & Axle Weight Legal Limit
Federal Bridge Formula B Explained: DOT Axle Weight Limits & Spacing Chart
Complete guide to Federal Bridge Formula B (23 CFR § 658.17): axle spacing mathematics, 80,000 lbs gross cap, steer and tandem limits, inner vs outer bridge rules, and sliding trailer tandems.
Cross-Border US & Canada Trucking: Weight Limits, HOS & Rate Conversions [2026]
Complete cross-border freight guide: US 80k Federal Bridge Formula vs Canadian MOU 39,500 kg tandem weights, FMCSA vs NSC Hours of Service, and currency conversion math.
Bridge Formula B Spacing: Kingpin to Rear Axle Measurement Guide [2026]
Comprehensive Kingpin to Rear Axle (KPRA) measurement guide. Learn state limits (California 40-ft rule, Florida 41-ft), tandem pin sliding, and bridge formula math.
Oversize & Overweight Permit Limits by State: 2026 Fleet Guide
Complete state-by-state oversize and overweight permit guide. Learn legal non-permitted thresholds (102" width, 13'6" height), pilot car escort rules, and spring thaw frost laws.
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Commercial Fleet & Freight Mathematical Verification
Federal Bridge Formula & Axle Weight Legal Limit Calculator
| Parameter / Operational Metric | Calculated Result |
|---|---|
| TRUCKERTOOLSHUB FEDERAL BRIDGE FORMULA B AUDIT REPORT | |
| Configuration | 5-Axle Commercial Combination |
| Outer Wheelbase Distance | 51 feet (51.0 ft) |
| Bridge Formula Max Capacity | 79,875 lbs |
| Federal Statutory Max Cap | 80,000 lbs (Without Oversize Permit) |
| LEGAL MAXIMUM GROSS ALLOWED | 79,875 lbs |
| CURRENT CAT SCALE WEIGHTS | |
| Steer Axle | 11,800 lbs [Limit: 12,000 lbs] -> LEGAL ✅ |
| Drive Tandem | 34,000 lbs [Limit: 34,000 lbs] -> LEGAL ✅ |
| Trailer Tandem | 33,400 lbs [Limit: 34,000 lbs] -> LEGAL ✅ |
| TOTAL GROSS VEHICLE WEIGHT | 79,200 lbs |
| OVERALL COMPLIANCE STATUS | 100% LEGAL & SCALE READY ✅ |
| Generated at TruckerToolsHub.com (FMCSA 23 CFR § 658.17 Compliant) | |