Trucking Laws · Nationwide · October 23, 2026

How Weigh Stations Work: Truck Weight Limits, Bypass and Inspections

Weigh stations are a familiar sight along the Interstate. The scale checks weight, but a stop can also include a look at the driver's paperwork and the truck itself. This guide explains the federal weight limits, how scales and bypass programs sort traffic and what happens during an inspection. It is general information, not legal advice.

What weigh stations are for

The Federal Highway Administration (FHWA) says authorizing the Interstate Highway System in the 1950s created an interest in preserving highways built with federal funds. Its Vehicle Size and Weight Team oversees state enforcement of truck size and weight standards, and each state must file a yearly enforcement plan and certification with FHWA.

Who has to stop

Stop rules come from state law and posted signs, so they vary. In California, Caltrans explains that Vehicle Code Section 2813 requires every commercial vehicle driver to stop for an inspection of size, weight, equipment and smoke emissions where the California Highway Patrol is inspecting and signs require it. Failing to stop is a misdemeanor there.

Federal weight limits

The federal rule for the Interstate System is 23 CFR 658.17. It sets three main limits:

• Gross vehicle weight: 80,000 pounds, unless the Bridge Formula calls for less
• Single axle: 20,000 pounds
• Tandem axle: 34,000 pounds

Under the definitions in 23 CFR 658.5, single axle weight covers wheels whose centers fall within 40 inches of each other, and tandem axle weight covers two or more consecutive axles whose centers are more than 40 inches and not more than 96 inches apart.

Tolerances, grandfather rights and permits

The rule says those limits include all tolerances. The one exception is an allowance for portable wheel load scales, never more than 5 percent.

Some states can go higher on the Interstate because of grandfather rights, weights that were legal under state law before the federal limits took effect. Off the Interstate, FHWA says states may set their own standards, and states can issue special permits for loads that cannot be divided. Federal law, 23 U.S.C. 127, raises the gross and axle limits for a truck with working idle reduction technology, such as an auxiliary power unit, by the weight of the unit, up to 550 pounds, and the driver must be able to show proof that it works. The older regulation text in 23 CFR 658.17 still lists 400 pounds, so check how each state on the route applies it.

The Bridge Formula in plain terms

FHWA says Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. The fix is either spreading weight over more axles or spacing axles farther apart. The formula is:

W = 500 (LN / (N - 1) + 12N + 36)

W is the maximum weight on a group of two or more consecutive axles, rounded to the nearest 500 pounds, L is the distance in feet between the outer axles of the group, and N is the number of axles in the group.

Examples from FHWA

FHWA's pamphlet walks through a common five-axle tractor and trailer:

• Axles 1 through 3 carry 46,000 pounds over 20 feet. The formula allows 51,000 pounds, so that group passes.
• Axles 2 through 5 carry 68,000 pounds over 35 feet. The formula allows 65,500 pounds, so that group is a violation even though every axle and the gross weight are legal.

There is one key exception. Two consecutive sets of tandem axles may carry 34,000 pounds each if the distance between the first and last axles of those tandems is 36 feet or more. FHWA says axle groups 1 through 3, 1 through 5 and 2 through 5 are the critical ones to check on that type of truck.

The free truck weight and axle calculator runs these checks. Enter steer, drive and trailer weights and axle spacing to compare them with the federal limits and the Bridge Formula.

How scales and bypass programs sort trucks

Many stations no longer weigh every truck on a static scale. FHWA describes weigh-in-motion (WIM) sensors embedded in the highway that weigh trucks at mainline speed as they approach a station. The estimate is compared with a pass or fail threshold set as a percent of the legal weight. Trucks over the threshold are sent into the station to be weighed on more accurate static scales, the ones used to write citations.

FHWA calls this electronic screening. Its stated intent is to let safe and legal trucks pass while enforcement focuses on high-risk carriers and vehicles. WIM is often part of that screening, checking weight at the same time as automated safety and credential checks.

PrePass and Drivewyze

Bypass services connect trucks to that screening. In general terms:

• PrePass says it uses both RFID transponders and a mobile app. Its FAQ says the officer at the station sees information such as the DOT number, license plate, equipment ID, vehicle weight and the green or red light decision.
• Drivewyze, which its website now calls Fleetworthy Weigh Station Bypass, says it partners with more than 40 telematics and ELD providers and offers owner operators a free 30-day trial on a phone or tablet.

FHWA says the driver is signaled to bypass or pull in before reaching the station ramp. PrePass says that when both its app and transponder are in the cab at a transponder site, the app tells the driver to follow the transponder.

What happens during an inspection

Roadside inspections happen at weigh stations, border checkpoints, bus terminals and traffic stops, according to the federal motor carrier agency's data glossary. Certified inspectors use North American Standard inspection levels:

• Level I (Full): the driver's license, medical certificate, record of duty status, hours of service, seat belt and inspection reports, plus a full vehicle check including brakes, coupling devices, lights, cargo securement, steering, suspension, tires and wheels.
• Level II (Walk-Around): the same driver items and the vehicle items that can be checked without getting under the truck.
• Level III (Driver Only): the driver items only, plus carrier identification.

Since June 23, 2025, under 49 CFR 391.41, CDL and CLP holders with a current medical certificate no longer have to carry the paper card. Drivers with a medical variance must still carry that paperwork.

Out of service

Under 49 CFR 396.9, inspectors can declare a vehicle out of service when its mechanical condition or loading makes it unsafe, and it cannot be driven until the required repairs are done. The driver must deliver the inspection report to the carrier, which has 15 days to certify the violations were corrected.

Tips for a smooth stop

• Know the weights before the scale does. Run the numbers in the truck weight and axle calculator, and remember that state, grandfathered and permit rules can differ.
• Do a full walkaround. The pre-trip inspection checklist covers the same brakes, lights, tires and coupling items a Level I inspection looks at.
• Keep logs in order. See hours of service and ELD rules and the hours of service break calculator.
• Follow the signs and signals. Open station signs and red or green bypass signals tell drivers what to do.
• Keep paperwork handy. The free trip log keeps each day's details in one place.

More free calculators are on the Tools page.

Frequently Asked Questions

What is the maximum legal weight for a semi truck?

On the Interstate System, 23 CFR 658.17 sets 80,000 pounds gross, 20,000 pounds on a single axle and 34,000 pounds on a tandem, and the Bridge Formula can require less. Grandfather rights, permits and state rules off the Interstate can change those numbers.

What is the Bridge Formula?

It is a federal formula, enacted in 1975, that limits the weight on any group of two or more consecutive axles based on the number of axles and the distance between the outer ones.

How do weigh station bypass programs work?

Services such as PrePass and Drivewyze link trucks to electronic screening, and a signal tells the driver to keep going or pull in.

What do inspectors check at a weigh station?

It depends on the level. A full Level I inspection covers the driver's license, medical certificate and logs, plus brakes, lights, tires, coupling devices and cargo securement.

What happens if a truck is put out of service?

Under 49 CFR 396.9, it cannot be driven until the repairs listed on the out-of-service notice are complete.

This guide is general information, not legal advice. Weight limits and stop rules vary by state, road and permit, so check the official rules for every state on a route.

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