Square and Rectangular Tubes Thickness Selection

What is the Thickness of Square and Rectangular Tubes?

Square tubes and rectangular tubes, also known as square or rectangular cold-formed hollow steel sections, are a common type of hollow structural steel. Their thickness is a key parameter that directly affects their performance and application range. The thickness of a square or rectangular tube, often called wall thickness, refers to the vertical distance from the outer surface to the inner surface of the tube; that is, the thickness of the tube wall. It is usually specified in millimeters, such as 2.0 mm, 3.0 mm, etc. This value represents the actual thickness of the material and is one of the important indicators for measuring the structural strength of square tubes.

In engineering drawings and procurement contracts, the tube wall thickness is a crucial core parameter that directly determines the square tube's:

1. Structural strength and load-bearing capacity: The thicker the wall, the greater the load (pressure, bending moment) the square or rectangular tube can withstand, and the more stable the structure.
2. Weight and cost: Wall thickness is a key factor in calculating the theoretical weight. Increasing the wall thickness significantly increases the weight per meter and the total cost
3. Application Scenarios: Square and rectangular tubes of different thicknesses are suitable for completely different fields, from lightweight decoration to heavy-duty load-bearing structures.

Thickness Measurement Methods: 

There are two main methods for measuring the thickness of square and rectangular tubes: one is actual measurement, using tools such as calipers or ultrasonic thickness gauges to directly measure the tube wall thickness; the other is based on standard specifications, confirming whether the thickness is qualified according to the tolerance range specified in the production standard. In practical applications, using both methods in combination can more accurately determine whether the thickness of the square tube meets the requirements.

Square and Rectangular Tube Thickness Range:

1) The following are common thickness ranges based on different production processes:

1. Welded Square and Rectangular Tubes

Thin-walled Square and Rectangular Tubes: This is the most common type of square tube, with the widest thickness range.

Common Range: 0.5mm ~ 3.0mm
Features and Process: Mostly made of cold-rolled steel strip through high-frequency welding (ERW), with precise dimensions and a smooth surface.
Applications: Furniture, interior decoration, display racks, light-duty shelves, appliance housings.

Medium-thick wall square and rectangular tubes:

Common range: 3.0mm ~ 6.0mm
Features and manufacturing process: Made from hot-rolled or cold-rolled steel strips, this is the mainstay of structural applications.
Applications: Building steel structures (beams, columns), machinery frames, vehicle manufacturing.

Thick-walled/Extra-thick wall square and rectangular tubes:
Common range: 6.0mm ~ 20.0mm or even thicker.
Features and manufacturing process: Usually produced using submerged arc welding (SAW) or rolled and welded from special thick plates.
Applications: Heavy-duty factories, offshore platforms, key bridge components, piling foundations.

2. Seamless Square and Rectangular Tubes

Common range: Approximately 2.0mm ~ 12.0mm.
Features: Seamless square and rectangular tubes are formed from seamless round tubes through cold drawing or cold rolling processes. Their wall thickness is limited by the original round tube's wall thickness. They are usually not made very thin, nor are extremely thick.
Applications: Mechanical structures and hydraulic equipment requiring high integrity and pressure resistance.

2) Common Thicknesses of Square and Rectangular Tubes in the Market

a. Small-sized square tubes (e.g., 20x20mm, 25x25mm, 30x30mm)
Common thicknesses: 1.0, 1.2, 1.5, 1.8, 2.0, 2.5, 3.0 mm. Thicknesses exceeding 3mm are less common.

b. Medium-sized square/rectangular tubes (e.g., 50x50mm, 100x50mm, 100x100mm)
Common thicknesses: 2.0, 2.5, 3.0, 4.0, 5.0, 6.0 mm
These are the mainstays for structural applications. 3.0mm, 4.0mm, and 5.0mm are the most commonly used.

c. Large-size square/rectangular tubes (e.g., 150x150mm, 200x200mm, 300x200mm)
Common thicknesses: 4.0, 5.0, 6.0, 8.0, 10.0, 12.0 mm

Due to the large cross-section, even a wall thickness of 4.0mm is considered thin-walled and prone to local buckling; therefore, a thicker wall thickness is usually required.

Factors Influencing the Selection of Square & Rectangular Tube Thickness:

1. Structural Mechanics Requirements—load-bearing capacity, stiffness, and stability

This is the most crucial factor determining thickness. Thicker square/rectangular tubes typically have higher structural strength (resistance to pressure and torque) and stability, capable of withstanding greater loads. For example, in building structures, thicker square/rectangular tubes are often chosen for sections requiring support of heavy loads to ensure safety and durability. Conversely, for light loads or decorative applications, thinner square/rectangular tubes may be more economical and practical.

2. Operating Environment and Durability Requirements

Corrosive Environments:
Indoor Dry Environments: Low corrosion risk; thickness can be selected according to mechanical requirements.
Outdoor, High Humidity, Coastal, or Chemical Environments: Corrosion allowance must be considered. This means adding 1-2 mm or more to the calculated theoretical thickness to ensure sufficient strength even if the material corrodes within the design life.
Abrasion Conditions: If the square or rectangular tube will be subjected to material erosion or mechanical wear, an increased wall thickness is required as an abrasion allowance.

3. Connection and Node Construction

Welding Requirements: If beveling welding is required at the tube ends or body, sufficient wall thickness is necessary to ensure weld strength and penetration depth.
Bolted Connections: If the tube wall needs to be drilled and subjected to bolt shear force, thin-walled tubes may tear or fail under pressure at the holes; a thicker wall thickness is required to ensure connection strength.

4. Processing Technology and Manufacturing Limitations

Cold Bending Formability: For welded square or rectangular tubes, the raw material (steel plate) needs to be cold-bent using a die. Thin walls are prone to wrinkling, while thick walls are difficult to form and require more powerful equipment. Each manufacturer has its own range of thicknesses it can produce.
Minimum Thickness Limitation: For large-section square and rectangular tubes, there is an empirical minimum wall thickness requirement for rigidity and stability.

5. Cost and Weight Optimization

Material Cost: Wall thickness is the most significant factor affecting the weight and price of square and rectangular tubes. While meeting all safety and technical requirements, the minimum wall thickness within the allowable range should be selected to achieve lightweighting and cost savings.
Life Cycle Cost: In corrosive environments, choosing slightly thicker tubes may have a higher initial cost, but it can reduce later maintenance and replacement costs, potentially making it more economical in the long run.

Material Selection: 

The choice of square and rectangular tube thickness is also influenced by the type of material. Common square tube materials include carbon steel, stainless steel, and aluminum alloys. Different materials have different mechanical properties and corrosion resistance, so even with the same thickness, their actual performance may vary. For example, stainless steel square tubes, due to their excellent corrosion resistance, may be more suitable for humid or corrosive environments than carbon steel square tubes of the same thickness.

Standardization of Square Tube Thickness:

Various domestic and international standard systems regulate the thickness and tolerances of square tubes, such as national standards and industry standards. These standards ensure the consistency and interchangeability of square tube products, providing a basis for user selection and use. Referring to relevant standards when selecting square tubes can help users better determine whether a product meets their requirements.

Standard Specifications for Square and Rectangular Tube Thickness Tolerances: 

The wall thickness of square and rectangular tubes is subject to strict national standards. When the wall thickness of a square or rectangular tube is no greater than 10mm, its permissible deviation is clearly limited, i.e., it must not exceed ±10% of the nominal wall thickness. If the wall thickness exceeds 10mm, the permissible deviation is adjusted to ±8% of the wall thickness. These precise standard specifications aim to ensure the safety and reliability of square and rectangular tubes during use.

Importance and Application Scenarios of the Specifications: 

As an important structural material, square and rectangular tubes are widely used in many fields such as construction, bridges, and machinery manufacturing. The allowable deviation in wall thickness directly affects the load-bearing capacity and service life of square and rectangular tubes. Therefore, strictly controlling wall thickness tolerances is crucial to ensuring project quality and safety. For example, in building construction, square and rectangular tubes are commonly used in the construction of supporting structures and load-bearing walls; the uniformity and stability of their wall thickness are vital to the overall stability of the building.

Special Considerations for Wall Thickness in Bends and Welded Areas:

It is worth noting that the above-mentioned allowable deviations in wall thickness do not include the wall thickness in bends and welded areas. This is because these areas play a special role in the structure of square and rectangular tubes, and their wall thickness often requires more precise control to ensure the overall strength and integrity of the structure. During production and quality inspection, these critical areas undergo additional testing and evaluation to ensure they meet higher standards.

Conclusion: 

In practical applications, the selection of square tube thickness requires comprehensive consideration of multiple factors. Firstly, there are the requirements of the application scenario, including load size, environmental conditions, and service life. Secondly, there are cost factors; thicker square tubes are generally more expensive, so a balance between economy and practicality must be struck while meeting performance requirements. Finally, there's the convenience of processing and installation. Thicker square tubes are generally heavier, making processing and transportation more difficult, while thinner square tubes are easier to handle and install.

Purchasing Considerations:

Thickness Tolerance: Square and rectangular tubes have permissible thickness deviations (positive and negative tolerances). For example, a nominal wall thickness of 3.0mm might actually be between 2.7mm and 3.3mm, which is still considered acceptable. For demanding applications, the wall thickness tolerance range (i.e., the actual thickness should not be less than the nominal thickness) must be clearly defined with the supplier.

Stability and Width-to-Thickness Ratio: In structural design, for compression members, the width-to-thickness ratio (section height/wall thickness) of the tube wall needs to be checked to prevent local instability. Standards have upper limits for the width-to-thickness ratio, which directly determines the minimum wall thickness required for a given cross-sectional dimension.

Frequently Asked Questions:

1. What is the relationship between the thickness of a square tube and its load-bearing capacity?

This is a common question. Simply put, thicker square tubes generally have higher structural strength and stability, and can withstand greater loads. For example, in building structures, thicker square tubes are often chosen for parts that need to support heavy loads to ensure safety and durability. Conversely, for light loads or decorative applications, thinner square tubes may be more economical and practical.

2. Is thicker square tube always better?

While thicker square tubes offer higher strength, excessively thick walls can add unnecessary weight and cost, and even lead to waste in some applications. The thickness of the square tube should be reasonably matched to actual needs to avoid over- or under-design.

3. What are the thickness tolerances for square and rectangular tubes?

According to national standards, the permissible deviation of the wall thickness of square and rectangular tubes should not exceed ±10% of the nominal wall thickness when the wall thickness is not greater than 10mm, and ±8% when the wall thickness is greater than 10mm. This regulation ensures the quality and safety of square and rectangular tubes, but it should be noted that this regulation does not apply to the wall thickness of bends and weld areas.

Read more: Square tube vs Round tube: Which is better? or Square Tube Size Chart in mm