Weight & Axle Distribution Calculator
Check if your loaded truck meets federal weight limits. Calculates gross vehicle weight, weight per axle, and flags overweight conditions.
Results
Visualization
How It Works
Federal weight limits for commercial vehicles on the Interstate System come from 23 USC §127 and apply through the Federal Bridge Formula codified in Title 23 USC §127(a) and Appendix C to Subchapter A of 23 CFR. The three governing ceilings: 80,000 lbs gross vehicle weight, 20,000 lbs on any single axle, 34,000 lbs on a tandem axle group. The Bridge Formula W = 500((LN/(N-1)) + 12N + 36) caps the gross weight on any group of two or more consecutive axles based on outer axle spacing (L in feet) and number of axles in the group (N). Bridge Formula B is what catches overweight trucks that pass the simple GVW check — a 5-axle combination with tight axle groupings can fail bridge formula well under 80,000 lbs.
The Formula
Bridge Formula B: W = 500 * ((L*N/(N-1)) + 12*N + 36)
where W = max gross weight in lbs on N consecutive axles, L = distance in feet between outer axles of the group, N = number of axles in the group
Federal axle limits: 20,000 single / 34,000 tandem / 42,000 tridem (typical state cap)
Variables
- GVW — Gross Vehicle Weight: total of tractor + trailer + cargo + driver + fuel + accessories
- 80,000 — 23 USC §127(a)(1) federal max GVW on Interstate System
- 20,000 — 23 USC §127(a)(1) max on any single axle
- 34,000 — 23 USC §127(a)(1) max on tandem axle group (axles spaced more than 40 in but less than 96 in apart)
- 12,000 — Industry standard steer axle rating (FMVSS 121 brake systems and tire load ratings); some states allow 14,000-15,000
- L — Bridge Formula: outer axle spacing in feet for axle group under analysis
- N — Bridge Formula: number of axles in the group
Worked Example
Standard 5-axle combination: 18,000 lb tractor, 14,000 lb trailer, 48,000 lb cargo = 80,000 GVW (at the federal limit). Now check bridge formula on the 4-axle group from steer to forward-most trailer tandem (L = 50 ft, N = 4): W = 500 x ((50 x 4/3) + 12 x 4 + 36) = 500 x (66.67 + 48 + 36) = 500 x 150.67 = 75,335 lbs. Group weight (steer 12,000 + drives 34,000 + first trailer axle 17,000) = 63,000 lbs — passes. If the trailer tandems are slid to the rearmost hole, drive axles often run 35,500-36,000 (over 34k limit) while trailer tandems drop to 30,000-31,000 — the truck passes GVW but fails the federal tandem axle limit. Sliding tandems forward 4 holes shifts approximately 1,600-2,000 lbs from drives to trailer.
Practical Tips
- Tandem slide approximation: each hole on a typical sliding-tandem trailer moves roughly 250-500 lbs between drive axles and trailer tandems, with hole spacing of 6 inches. Slide forward = weight onto trailer tandems. Slide rearward = weight onto drive axles.
- CAT Scale (catscale.com) charges $13.50 for the first weight, $3.50 for re-weigh after sliding tandems within an hour. The Weigh My Truck app integrates with CAT for paperless re-weighs at terminals nationwide.
- Bridge formula failure shows up most often on short-wheelbase 5-axle setups (sliding tandems all the way forward) and on overweight permit loads with closely-spaced tridems. Calculate before loading, not at the scale house.
- Overweight violation penalties under state code: Ohio §4513.34 charges $80 + $0.05/lb over for first 2,000 lbs over, escalating; Texas Transportation Code §621.506 reaches $5,500 for grossly overweight. Federal civil penalty under 49 USC §31144 can reach $16,864 per violation 2024-adjusted for federally-enforced violations.
- State exceptions: Michigan allows up to 164,000 lbs GVW on certain configurations under MCL §257.722; Wyoming permits 117,000 lbs on grandfather routes. Toll roads and turnpikes (Indiana, Ohio, NJ Turnpike, Mass Pike) often have separate permit systems above federal limits.
- Soft suspension and worn airbags cause weight to creep onto drive axles between weighings. Have suspension inspected if you fail axle limits while passing GVW, especially at the same loading dock pattern repeatedly.
- Liquid loads in non-baffled tankers can surge during braking or cornering, momentarily overloading the steer axle by 1,500-3,000 lbs. Baffled trailers reduce surge to 200-400 lbs of dynamic shift; food-grade smooth-bore tankers (no baffles allowed for sanitation) require slower deceleration habits.
Frequently Asked Questions
What is the maximum weight for a semi-truck under federal law?
23 USC §127(a)(1) sets 80,000 lbs gross vehicle weight on the Interstate System. The same statute caps single axles at 20,000 lbs and tandem axle groups at 34,000 lbs. The Federal Bridge Formula in 23 USC §127(a)(2) further constrains weight on multi-axle groups based on outer axle spacing — many configurations cannot legally hit 80,000 lbs because bridge formula caps them lower.
What is the Federal Bridge Formula and why does it matter?
Bridge Formula B (codified at 23 USC §127(a)(2) and FHWA regulations): W = 500((LN/(N-1)) + 12N + 36). It caps gross weight on any group of consecutive axles based on outer spacing (L in feet) and axle count (N), preventing weight concentration that would damage bridges. A 5-axle truck running 80,000 lbs needs at least 51 ft of outer axle spacing (steer to rear trailer axle) to pass the bridge formula on the full vehicle. Tighter wheelbases mean lower legal GVW even if axle limits look OK.
What are the state-specific axle weight rules I need to know?
State variations: California limits steer axle to 12,500 lbs; Oregon allows 600 lbs/in of tire width; Florida allows 20,000 lb single but only 22,000 lb on any axle within tandem if axles spaced over 8 ft. Truckers Atlas and the FHWA size and weight database both maintain state-by-state matrices. State permits (NTOL, OS/OW) override default limits but require routing.
How do I weigh and adjust before going to the scale house?
CAT Scales at most truck stops: drive on, three-platform scale reads steer / drive / trailer simultaneously. Cost $13.50, ~3 minute turnaround. Slide tandems if drives or trailers exceed limits. Re-weigh within 1 hour for $3.50. State scale houses use static or weigh-in-motion sensors at posted exits along Interstates; bypass via PrePass or Drivewyze if you have a clean CSA score and properly registered transponder.
What happens if I get caught overweight?
Roadside enforcement: officer issues citation, vehicle may be placed out of service until weight redistributed or shed. State per-pound fines vary widely — Indiana charges $0.05/lb over the first 1,000 plus penalties; Pennsylvania escalates to $0.15/lb after 5,000 over. CSA points hit the carrier under Vehicle Maintenance BASIC and sometimes Driver Fitness. Repeat violators face out-of-service orders under 49 CFR §385 audit triggers.
What permits allow exceeding federal weight limits?
Each state issues its own oversize/overweight permits (NTOL — non-divisible load, divisible-load permits, harvest permits, ag exceptions). Federal exception 23 USC §127(j) allows 88,000 lbs for certain Auxiliary Power Units. Specialized hauling (windmill blades, transformers) routinely runs 120,000-200,000+ lbs under superload permits with route surveys, escort vehicles, and posted bridge analysis.
How does cargo placement affect axle weight distribution?
Center of gravity location relative to king pin determines weight split. Cargo loaded forward (within 8-12 ft of king pin) puts more on drive axles. Cargo loaded rearward (toward back of trailer) loads trailer tandems. Approximate rule: every foot forward of king pin shifts about 400 lbs from trailer to drives at 53-ft trailer length. Heavy single items (paper rolls, coils) need to land between tandems and king pin to balance both axle groups.