How Much Does a Semi Truck Weigh? Axle Limits, GVWR & Bridge Formula

A standard semi-truck typically weighs between 30,000 and 36,000 pounds empty (tare weight) depending on tractor and trailer specifications, and can legally weigh up to 80,000 pounds gross vehicle weight (GVW) on interstate highways without a special permit.

This means the usable payload capacity for most dry vans, reefers, and flatbeds falls in the 43,000–48,000 pound range, depending on tare weight and axle configuration.

For compliance, drivers must balance three key constraints:

• Axle limits: 20,000 lb single, 34,000 lb tandem under federal law.
• Bridge Formula: Ensures weight is distributed properly over axle spacing.
• Overweight permits: Required if exceeding federal/state weight caps.
• Scaling best practices: Regular weigh-ins, proper load distribution, and adjustments at CAT scales.

Key Terminology Drivers and Inspectors Use

Federal Baseline: The Numbers Every Interstate Driver Must Know

80,000 lb GVW Cap on the Interstate System

The federal gross vehicle weight (GVW) limit for combination vehicles operating on the Interstate System is 80,000 pounds. This cap applies unless the Federal Bridge Formula yields a lower allowable weight for one or more axle groups on your specific configuration. In practice, that means you can be under 80,000 lb and still receive a weight citation if an axle group (for example, your inner bridge group) exceeds its Bridge Formula maximum.

Axle Group Limits That Always Apply

• Single axle (≤ 40 in. between axle centers): 20,000 lb
• Tandem axle (> 40 in. and ≤ 96 in. between axle centers): 34,000 lb

These axle caps are federal baselines on the Interstate System and they supersede the Bridge Formula for the specific spacings noted:

• If axle centers are ≤ 40 in., the single-axle limit of 20,000 lb controls.
• If axle centers are > 40 in. and ≤ 96 in., the tandem limit of 34,000 lb controls.

Outside those spacings (e.g., spread tandems), the Bridge Formula governs the maximum weight.

“Grandfather” Rights and Intrastate Variations

When Congress set federal weight limits (and later amended them), some States retained higher limits under so-called “grandfather” rights. As a result:

• On certain Interstate routes in some States, legal limits may be higher than the baseline numbers above.
• Intrastate (non-Interstate) networks are governed by state weight laws and posted structures, which can be more restrictive or allow different axle/spacing combinations.
• The practical takeaway: plan your route with state-specific weight maps and be prepared for posted bridge restrictions that override general limits.

APU / Idle-Reduction Allowance: Up to +550 lb

Vehicles equipped with an auxiliary power unit (APU) or idle-reduction technology may exceed applicable federal weight limits by up to 550 lb - but only with proper proof. To claim this allowance:

• Carry written certification of the APU’s weight, or be able to demonstrate/certify the idle-reduction equipment is fully functional.
• Present documentation at inspection if requested.
  This allowance helps offset the equipment weight so drivers can reduce idling without losing payload.

Statute and Regulation Always Control (and the 2015 FHWA Notice)

For federal Interstate limits, the controlling law is 23 U.S.C. §127 and 23 C.F.R. §658. FHWA’s May 2015 guidance pamphlet on Federal Bridge Formula Weights supersedes earlier pamphlets and explicitly notes that it paraphrases the statute and regulation for illustration only. In any dispute, the statute and regulation take precedence. Treat the pamphlet - and any secondary summaries - as explanatory aids, not the final word.

Why the Bridge Formula Exists (and When It Controls)

Brief History: Protecting Bridges With a Weight-to-Length Rule

As trucks became heavier in the 1950s–1970s, Congress enacted the Bridge Formula (1975) to reduce structural stress on bridges. Rather than just capping total GVW, the law links allowable weight to both the number of axles and the spacing between them. The goal is simple: spread the load so that bridge members and pavements aren’t overloaded by concentrated axle groups.

The Key Concept: Same GVW, Different Stress

Two trucks can both weigh 80,000 lb:

• A longer wheelbase tractor–trailer spreads that weight over more distance, reducing peak stresses on bridge members.
• A shorter wheelbase truck concentrates the same weight in a smaller area, creating higher stresses and potential damage.

This is why the Bridge Formula sometimes limits a shorter truck to a lower legal gross weight than a longer truck - even on the same route.

The Bridge Formula Itself (How It’s Applied)

Formula:
W = 500 × ( (L × N) / (N − 1) + 12N + 36 )

Definitions:

• W = maximum allowable weight (in pounds) on any group of two or more consecutive axles (rounded to the nearest 500 lb).
• L = distance in feet between the outer axles of the group being checked.
• N = number of axles in the group being checked.

Scope of application:

• You must apply the formula to every qualifying axle group (any 2+ consecutive axles) on the combination.
• Compliance is required for all such groups, not just the whole-vehicle “outer bridge” group. It is entirely possible to be legal on single axles, tandems, and total GVW, yet violate the Bridge Formula on an inner group.

How axle caps interact with the formula:

• For axles ≤ 40 in. apart, the single-axle cap (20,000 lb) replaces the formula result.
• For axles > 40 in. and ≤ 96 in. apart, the tandem cap (34,000 lb) replaces the formula result.
• For wider spacings (e.g., spreads) and for larger groups (e.g., “outer bridge”), you must use the formula to compute the maximum.

Operational takeaway: The formula is not just about GVW. It’s about where the weight sits. Short or tightly spaced groups will often be the limiting factor, even if your gross is under 80,000 lb.

The Critical Groups Inspectors Check Most on a 5-Axle Set

On a typical five-axle tractor–semitrailer (steer + drive tandem + trailer tandem), experience shows three groups are most likely to be limiting:

1. Axles 1–3 (the “tractor bridge” group)
   • This group covers the steer axle and the drive tandem.
   • Problems here often arise from heavy front loading (e.g., dense freight forward), a forward fifth-wheel setting, or full fuel affecting steer/drive balance.
2. Axles 2–5 (the “trailer/inner bridge” group)
   • This inner group spans the drive tandem through the trailer tandem.
   • It is a common source of violations when heavy freight is concentrated over the drives and the forward portion of the trailer, and when the distance L (between axle 2 and axle 5) is relatively short.
3. Axles 1–5 (the “outer bridge” group)
   • This is the entire combination from the steer to the rearmost trailer axle.
   • While the outer group often aligns with the 80,000 lb cap on standard lengths, compliance still depends on L and N. Certain wheelbase/trailer length combinations can yield an outer bridge limit below 80,000 lb, especially on shorter vehicles.

Why these three groups?
Although the formula applies to every 2-axle and larger combination, these three groups - 1–3, 2–5, and 1–5 - tend to produce the lowest allowable weights on standard 5-axle rigs. If you are compliant on these, the remaining groups are usually compliant as well (but don’t skip the full check when you’re close to the margins).

Overweight & Over-Axle Permits (Non-Divisible Loads)

Not every heavy load qualifies for an overweight permit. The deciding factor is whether the cargo is classified as non-divisible. A non-divisible load is one that cannot be reduced in weight or dimensions without destroying its intended function, risking damage, or requiring excessive cost and time. In practice, this means something that cannot be broken down during ordinary loading and unloading; states do not expect multi-day disassembly with specialized tooling.

By contrast, divisible loads - palletized goods such as paper, grain, coils, beverages, or lumber - are almost always ineligible. If a shipment can be split, repacked, or resequenced to meet legal weight requirements, it will not qualify for a permit. Some states run narrow “commodity programs,” but these are exceptions rather than the rule.

Equipment weight also matters. Chains, tarps, and toolboxes add to gross and axle totals, but permits never excuse exceeding the manufacturer’s GAWR or GVWR. The commodity itself is what defines divisibility.

Permitting Variables

Even with a permit, movement is subject to multiple restrictions. Routes must be approved for bridge capacity, and some corridors require engineering reviews for very heavy axle or gross weights. Jurisdictions frequently limit travel during peak traffic, weekends, or holidays, and urban centers often restrict moves to daylight hours only. Weather can also suspend operations: high winds, snow, or reduced visibility void movement authority until conditions improve.

Additional requirements may include escorts, police details, or bucket trucks to handle overhead wires. States set exact standards for banner sizes, flag placements, and amber lighting for night operations. Speed is often capped at 45 mph, and lane restrictions apply on bridges or ramps.

Operational Costs

Overweight movements are expensive not only because of permit fees but also due to routing, escorts, and staging requirements. Missed windows can cascade costs across the entire logistics chain. Pre-planning is therefore critical: a complete packet with verified axle weights, route clearance, and scheduled escorts can mean the difference between a seamless move and multi-day delays.

When Permits Don’t Help

Permits are not a cure-all. They cannot override manufacturer safety ratings or excuse defective equipment. Local bans, seasonal restrictions, or posted bridge limits can still apply even if a state issues a general permit. Above all, divisible loads do not qualify simply because the shipper overloaded a trailer or refused to break down a shipment.

Scaling Workflow to Avoid Fines and Downtime

A disciplined approach to weighing and adjustment is the driver’s best defense against fines and downtime. The workflow begins before loading: verify tare weight with fluids and gear, check whether your trailer has sliding tandems or lift axles, and review your route for posted restrictions.

At the dock, load placement is critical. Dense freight concentrated at the nose risks exceeding tandem limits; interleaving heavy and light pallets helps balance weight. Blocking and bracing prevents shifts that can move thousands of pounds between axles during transit.

Once loaded, the CAT scale process provides the definitive check. The first weigh ticket establishes steer, drive, tandem, and gross totals. From there, compare against the 12k/34k/34k rule of thumb and test bridge groups 1–3, 2–5, and 1–5. Adjustments include sliding tandems (affecting drives vs. trailer), moving the fifth wheel (affecting steer vs. drives), or, when necessary, repositioning cargo. Every meaningful change should be followed by a reweigh ticket.

Supporting paperwork is essential. Keep the APU documentation for the +550 lb allowance, retain both initial and reweigh tickets, and have equipment ratings available in the cab. When permits are in play, carry the documents, maps, and escort contact numbers required. On-board scale systems and calibrated suspension gauges can help prevent surprises, but the certified scale ticket remains the final word.

Frequent Violations (and How to Bulletproof Your Operation)

Legal on Gross, Illegal on Inner Bridge (the Classic Trap)

It is entirely possible to be ≤ 80,000 lb GVW and still be overweight on a Bridge Formula group, most commonly the 2–5 inner-bridge on a standard five-axle. Why it happens:

• Tight spacing (short L) between axle 2 and axle 5 paired with dense freight positioned forward of the trailer midpoint.
• Trailer tandem too far forward. Sliding rearward increases L for the 2–5 group and raises the allowed W (when the Bridge Formula controls), while shifting some actual weight from drives to trailer.
• No recheck after a slide. Drivers fix the drive/tandem split but forget the Bridge Formula check.

Bulletproofing tactics

• Train to always evaluate 1–3, 2–5, and 1–5 after the first weigh, not just 12k/34k/34k.
• Keep a laminated bridge quick-reference for your equipment’s common L values or use a calculator app.
• Record pounds-per-hole for each trailer in the fleet and train drivers to estimate the number of holes required before reweighing.

Tandem at 34,200 lb on Federal Interstates: The “Tolerance” Myths

Many drivers assume there’s always a built-in “tolerance” (e.g., a few hundred pounds) on federal interstates. In reality:

• Federal tandem limit is 34,000 lb (for axles spaced >40 in. and ≤96 in.). Some states apply small scale tolerances or enforcement policies, but these vary and may not exist where you’re traveling.
• Even where a tolerance policy exists, an officer can still cite for exceeding the statutory limit, especially if you’re appreciably over or if other issues are present (equipment condition, documentation).

Bulletproofing tactics

• Plan to be at or under 34,000 lb on tandem and 20,000 lb on a single axle. Treat “tolerances” as not guaranteed.
• If you must push margins, carry recent, clean scale tickets and ensure tires/axles match their GAWR and inflation specs so you’re not stacking violations.
• When close, slide the tandems one hole and reweigh rather than gambling on a rumored buffer.

Over on Steer After Fueling or Sliding the Fifth-Wheel Too Far Forward

Steer violations often appear after fueling or an aggressive fifth-wheel move:

• Full fuel tanks can add hundreds of pounds to the steer.
• Moving the fifth-wheel forward increases steer weight (helpful if steer was light) but can push you over 12,000 lb or over the steer GAWR or tire rating.

Bulletproofing tactics

• Know your tractor’s steer GAWR and tire load ratings and fuel with those limits in mind.
• Use conservative fifth-wheel moves: one notch at a time; reweigh.
• If you need a heavier steer for handling, consider higher-rated steer tires/axle during spec'ing so you maintain legal headroom.

Ignoring Posted Bridge Signs on Local Roads

Posted bridges/roads (weight, axle, speed) override general federal or state limits. Violating a posting can result in:

• Immediate citations, forced offload, or detour.
• Potential civil liability if infrastructure is damaged.

Bulletproofing tactics

• Route planning must include posted structure data, not just height and HAZMAT restrictions.
• Keep current state permit books/apps and ensure dispatch tools are configured to avoid restricted segments.
• If GPS tries to route you over a posted bridge, stop and reroute - do not assume permissibility because you’re “close enough.”

Load Shift After Hard Braking/Rough Roads - Prevention Even in Vans

A “legal at departure” load can shift and produce an axle or inner-bridge overweight mid-trip. Causes:

• Insufficient blocking/bracing or no friction mats on smooth floors.
• Empty space at the tail allowing pallets to migrate rearward on grades or during emergency stops.
• Humidity/condensation in reefers reducing friction between pallets and floor.

Bulletproofing tactics

• Use load bars, straps, or friction mats when allowed and appropriate.
• Fill gaps with dunnage; do not leave large voids where inertia can move freight.
• For mixed freight, interleave dense pallets between lighter ones to reduce big CG swings.
• After emergency braking or severe road events, reweigh at the next opportunity if you were near limits.

Understanding how much a semi-truck weighs is not just a matter of curiosity - it is a cornerstone of safe, legal, and profitable trucking. Between the federal 80,000-lb gross cap, axle group rules, and the Bridge Formula, drivers and carriers must constantly balance equipment choices, load placement, and routing decisions to stay compliant. Real-world tare weights of tractors and trailers set the stage for how much freight can legally and safely be hauled, while permits and scaling practices define how operations flow day to day.