Most states collect taxes based on the weight ­of transported goods. Truck weigh stations go toward those taxes, but they are also important to monitor the weight of a truck to ensure that it falls within the safety guidelines that each place has set for its road system.

So, how much does a semi truck weigh? We'll explore what a semi truck weighs and how weigh stations work.

Contents

1. Why Weighing Trucks Is Important
2. What a Semi Truck Weighs
3. How Semi Truck Weigh Stations Work

Why Weighing Trucks Is Important

The weight of a semi truck is a critical factor that impacts its performance and safety. Every semi has a Gross Vehicle Weight Rating (GVWR) — the maximum weight at which a vehicle can safely operate. This rating includes the weight of the truck, trailer, cargo, and even passengers.

Exceeding this weight can lead to safety issues for truck drivers and others, as well as legal consequences.

What a Semi Truck Weighs

You can either measure the unladen weight, which is the total weight of the tractor and trailer of an unloaded truck. An unladen truck can weigh between 10,000 and 25,000 pounds (4,535 to 11,339 kilograms).

Or, you can measure the laden weight, which is what you get when you factor in semi-truck weight and trailer weight as well as the cargo weight of a loaded trailer.

The GVWR also determines what classification a vehicle falls under.

• Class 1: Light-duty vehicles weighing up to 6,000 pounds (2,722 kilograms). This includes SUVs and minivans.
• Class 2: Light-duty vehicles, like medium-size pickup trucks, that weigh between 6,001 (2,722 kilograms) and 10,000 pounds (4,536 kilograms).
• Class 3: Light-duty or medium-duty vehicles, like smaller box trucks or medium-duty trucks (which can also fall into other categories), weighing up to 14,000 pounds (6,350 kilograms).
• Class 4: Medium-duty vehicles, like a small dump truck, that weigh between 14,001 pounds (6,350 kilograms) and 16,000 pounds (7,257 kilograms).
• Class 5: These medium-duty vehicles, like a large maintenance truck, weigh between 16,001 pounds (7,257 kilograms) and 19,500 pounds (8,845 kilograms).
• Class 6: Medium-duty vehicles include a medium school bus and a large step van. Vehicles in this category weigh between 19,5001 pounds (8,845 kilograms) and 26,000 pounds (11,793 kilograms).
• Class 7: Categorized as heavy-duty, this class includes triple-axel vehicles. City buses and garbage trucks weighing between 26,001 pounds (11,793 kilograms) and 33,000 pounds (14,969 kilograms) fall into this classification.
• Class 8: This class includes heavy-duty trucks that weigh 33,001 pounds (14,969 kilograms) and over. Tractors and coach buses fall in this category.

How Semi Truck Weigh Stations Work

Truck scales can handle an enormous amount of abuse. Steel, concrete or, in most cases, a combination of both, make up the scales. The technology used in the scales to measure fully loaded semi trucks and other large vehicles varies.

Load-cell systems are the most popular technology used. Each cell is comprised of a durable material such as steel or concrete with one or more strain gauges attached to or embedded in it. A strain gauge consists of a wire (or wires) that transmits a mild electric current.

As the cell is subjected to weight, the wire in the strain gauge is altered or compressed slightly. The change in the wire results in a difference in the resistance to the current passing through it. The signal from each cell is sent to a junction box, where sensors measure the variance in the current and calculate the amount of weight the scale is supporting.

The strain gauges in load cells can be either compression- or tension-based. A compression strain gauge depends on how much the cell compresses when pressure is applied, while a tension strain gauge is on the slight change in the shape of the cell caused by the weight.

A bending-plate system uses metal plates with strain gauges attached to them. As weight is applied to the scale, the plates are subjected to stress. The strain gauge on each plate measures the amount of stress and calculates the load required to cause it. The amounts from each gauge are added together to get the total for that axle.

Piezoelectric systems use a series of piezoelectric sensors. The sensors are embedded in a conducting material.

When weight is applied, the pressure changes the voltage of the electrical charge flowing through the conductor. The sensors measure the change in voltage and calculate the load. The amounts from each sensor are added together.

In addition to the different scales used, there are at least three methods of getting the weight of a truck:

• One-axle: The most cumbersome method, a truck gradually drives across a single scale, stopping each time a set of wheels is on the scale. Once all the axles have been weighed, the total is added together.
• One-stop: A series of scales are used so that the entire truck can be weighed at once. The scales are typically connected to a single electronic controller that automatically combines the axle weights to get the gross weight.
• Weigh-in-motion (WIM): WIM uses a series of embedded sensors to calculate the weight per axle as a truck drives over the sensor pad. Unlike the other two methods, there is no need for the truck to come to a complete stop while on the scales. In fact, some WIM systems are on highways so that all traffic is monitored at speed.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.