As a construction professional, you rely on the stated specifications of building materials to be accurate for planning and safety. However, have you ever purchased a 1 1/2-inch scaffold tube only to discover it does not actually measure 1 1⁄2 inches in diameter? This common occurrence can be frustrating and even dangerous if not properly understood. The reasons behind this misleading labeling come down to manufacturing processes, industry standards, and the complex relationship between a tube’s outer diameter, inner diameter, and thickness. Before purchasing or working with scaffold tubes, you should be aware of how sizing conventions can differ from actual measurements to ensure you choose components suitable and safe for your needs. This article will explain the logic behind scaffold tube sizes so you have the full picture of your work.

The History Behind Scaffold Tube Sizing

The sizing of scaffold tubes can be misleading. Typical scaffolding dimensions “1 1⁄2 inch” scaffold tube, the actual outside diameter of the tube is closer to 1 3/8 inches. This discrepancy originates from the historical development of scaffold tube sizing.

Originally, scaffold tubes were sized according to their inside diameter. A 1 1⁄2 inch scaffold tube had an inside diameter of 1 1⁄2 inches, with a larger outside diameter required to have that inside measurement while still maintaining adequate wall thickness. Ensuring components like boards, brackets, and fittings could be attached inside the tube was the main objective of this inside dimension.

Over time, the scaffold tube sizing system transitioned to referencing outside diameters instead. However, the naming convention of “1 1⁄2 inch tube” was already ubiquitous. Rather than introduce confusion by relabeling the tube “1 3/8 inch,” the original naming system persisted. The result is a scaffold tube whose name reflects its historical inside diameter, not its actual outside diameter.

This seemingly small difference of 1/8 inch has significant implications for scaffold construction. One must properly size pipework, couplers, brackets, and other fittings to fit the outside diameter of 1 3/8 inches. However, boards and platforms can still be sized to the inside diameter of 1 1⁄2 inches. One must exercise caution to ensure all components are sized and rated properly for the application.

While perhaps not the most logical system, the scaffold tube sizing method has endured for decades due to its familiarity. Understanding the rationale behind this seemingly misleading label can help prevent confusion when working with or specifying scaffold tubes. With the proper knowledge and component selection, people can safely use 1 1/2 inch scaffold tube for generations to come.

How Scaffold Tubes Are Manufactured

Standard scaffold tube diameter with 1 1⁄2 inch tube does not have a 1 1⁄2 inch diameter. It is important to understand how these tubes are manufactured.

The process of hot rolling is used to make scaffold tubes by heating a long cylindrical billet of steel and passing it through a series of rollers that reduce its diameter and increase its length. The compression of the billet decreases its diameter while the wall thickness remains consistent. The final diameter of the tube is determined by how much the billet is compressed, not its initial diameter.

Tube sizes refer to the nominal bore, not the outside diameter

The sizes of scaffold tubes refer to their nominal bore. which is the approximate inner diameter of the tube, not the outside diameter. So a 1 1⁄2 inch tube will have an inner diameter of scaffold tube  close to 1 1⁄2 inches, but an outer diameter significantly greater than that to account for the wall thickness. The wall thickness depends on the grade of steel used and the pressures the tube needs to withstand.

Higher grades of steel and increased pressures require thicker tube walls. which results in a larger difference between the inner and outer diameters. For most scaffold tubes, the outer diameter can be 1/4 inch to 3/4 inch greater than the nominal bore. So a 1 1⁄2 inch tube may have an outer diameter closer to 2 inches. The exact measurements can vary between manufacturers and steel grades.

In summary, scaffold tube sizes denote their nominal internal diameter, not their external diameter. The manufacturing process used to produce the tubes and the grades of steel determine their final outer dimensions. So while a 1 1⁄2 inch scaffold tube has an approximate inner diameter of 1 1⁄2 inches, its outer diameter will be significantly larger to provide the necessary wall thickness.

The Difference Between Nominal Pipe Size and Actual Outside Diameter

The nominal pipe size (NPS) refers to the rough commercial sizes that have been adopted for standard pipe and fittings. The actual outside diameter (OD) of a pipe is often larger than its nominal size. This difference occurs because pipe dimensions are specified by a standard for a particular pipe material and schedule (wall thickness).

Standard Sizing

Most pipe and tube is specified and dimensioned according to a series of long-established standard sizing systems. The most common standards are the Iron Pipe Size (IPS) and the Copper Tube Size (CTS). These were created decades ago and are still used today to specify and size plumbing pipe and tube for buildings.

Wall Thickness

The wall thickness of a pipe, also known as the pipe schedule, can vary and is specified separately from the nominal pipe size. As the schedule increases, so does the wall thickness. Thicker walls provide more durability and higher pressure ratings, but reduce the inner diameter and volume. Standard schedules are Schedule 40 and 80 for steel pipe.

The actual measured outside diameter of 1 1⁄2-inch Schedule 40 steel pipe is 1.900 inches. The wall thickness for Schedule 40 pipe is 0.133 inches, giving an inside diameter of 1.634 inches. The nominal size only roughly corresponds to the actual OD, and does not indicate the inner diameter or wall thickness.

Why the Difference?

The historical origin of the difference between nominal pipe size and actual diameter is to provide compatibility between pipes and pipe fittings of the same nominal size, even when their dimensions are not exactly the same. The variation in dimensions also allows for manufacturing tolerances. Nominal sizing also continues for the purpose of compatibility with older pipe systems.

In summary, the nominal pipe size is for identification and compatibility purposes, whereas the actual outside diameter and wall thickness are the dimensions that physically define the pipe. Knowing the difference between these two measurements and standards is important for proper pipe selection and installation.

Why the Size Discrepancy Matters for Construction

When purchasing scaffolding tube, it is important to understand why the stated size of a tube, such as 1 1⁄2 inches, does not correspond to its actual outside diameter. This discrepancy occurs due to manufacturing processes and ensures safe, consistent fittings between tubes and connectors.

Tube Wall Thickness

Scaffolding tubes are made of steel, which requires a certain thickness to maintain structural integrity under heavy loads. The wall thickness of a tube reduces its internal diameter, so a 1 1⁄2 inch tube may have an outside diameter closer to 2 inches to accommodate a wall thickness of 1⁄4 inch. The wall thickness depends on the tube’s load rating to ensure it can support the required weights.

Fittings and Connectors

Scaffolding systems rely on tight-fitting connectors and clamps to join tubes together securely. If tubes had an outside diameter exactly matching their stated size, these fittings would not fit properly. The additional diameter provided by the wall thickness gives fittings more surface area to grip onto, preventing instability.

Industry Standards

Manufacturing scaffolding tubes to standard outside diameters allows for interoperability between components from different makers. This standardization benefits end users by providing more flexibility and compatibility in scaffolding systems. Although the stated size of a tube may differ from its measurements, the predictable variation gives scaffolding a universal usefulness for construction and industrial purposes.

In summary, the reason a 1 1⁄2 inch scaffold tube does not have a precisely 1 1⁄2 inch diameter relates to essential factors of material thickness, component compatibility, and industry standards. While the nomenclature may seem misleading, the predictable size discrepancy results in a safe, standardized, and highly functional product for scaffolding applications. Buyers should keep these reasons in mind when selecting and purchasing scaffolding materials.

How to Correctly Measure and Order Scaffold Tubes

When ordering scaffold tubes, it is important to understand how the dimensions are properly measured to ensure you receive the correct sizes. The nominal diameter of a tube, like 1 1⁄2 inches, refers to its approximate outer diameter. However, the actual outer diameter will be slightly larger to account for the tube wall thickness.

How to Measure

To determine the actual outer diameter, you will need to measure the tube with a precision instrument like digital calipers. Measure at several points around the circumference of the tube end to get an accurate reading, as diameters can vary slightly. For a 1 1⁄2 inch nominal tube, the outer diameter should measure around 1.660 to 1.720 inches.

The inner diameter, or bore, is calculated by subtracting the wall thickness from the outer diameter. Wall thickness can range from 0.125 to 0.188 inches for scaffold tubes. To find the wall thickness, measure from the inner to outer surface at the tube end. For example, a 1 1⁄2 inch tube with a 0.160-inch wall thickness would have an outer diameter of 1.720 inches and an inner diameter of 1.400 inches.

Why the Difference?

The difference between the nominal and actual diameters exists to allow for proper fitment and safety. The extra outer diameter provides enough material for the tubes to securely join together, while the specified wall thickness gives the tube adequate strength. The inner diameter must be large enough to allow unrestricted flow or insertion of items through the entire length of the tube.

Ordering the Correct Size

When purchasing scaffold tubes, be sure to order by the actual outer diameter and specify the wall thickness required for your needs. Give the vendor the precise measurements to ensure you receive tubes that will fit together properly and suit the loads they need to bear. Trying to force mismatched tubes together or using tubes that are too thin for the application can lead to structural instability and collapse. By understanding sizing conventions and taking accurate measurements, you can confidently order scaffold tubes in the proper dimensions for your project.


By understanding the key reasons why a 1 1/2 inch scaffold tube does not actually measure 1 1/2 inches in diameter, you now have valuable insight into the scaffolding industry and manufacturing processes. While it may seem counterintuitive or misleading at first, the dimensional standards and tolerances in place have important reasons and purposes. Appreciating these nuances will make you a more informed consumer and help set proper expectations when purchasing or working with scaffolding materials. Though a small detail, it illustrates how even the most seemingly straightforward parts of an industry can have layers of complexity not visible from the outside. With this newfound knowledge in hand, you can approach your next worksite or scaffolding project with more confidence and expertise.