Identifying the correct size of tubing is one of those tasks that sounds simple until you are actually standing in front of a piece of pipe, trying to figure out what you need to order. Whether you are sourcing materials for a vessel repair, an offshore installation, or an industrial project, getting the size right the first time matters. A wrong dimension can delay an entire job, and in maritime and offshore environments, delays are expensive. This guide walks you through exactly how to tell what size tubing is, what the measurements mean, and what to watch out for.
What does tubing size actually refer to?
When someone refers to a tubing size, they are almost never talking about a single measurement. Tubing size is a combination of three values: the outer diameter (OD), the inner diameter (ID), and the wall thickness. Together, these three dimensions define the tube completely and determine whether it will fit, flow correctly, and handle the required pressure.
In most industrial and maritime contexts, the nominal size is also used as a reference. This is a standardized label that does not directly correspond to any actual physical measurement. It exists to simplify ordering and communication across suppliers and standards. So when you see a tube listed as “1 inch,” that label is a nominal designation, not a guarantee that the outer diameter is exactly one inch.
Understanding this distinction is the foundation of getting tube sizing right. Once you know what each number refers to, the rest becomes much more straightforward.
What’s the difference between pipe sizing and tube sizing?
This is where a lot of confusion begins. Pipes and tubes are measured differently, and the two systems are not interchangeable.
For pipes, the sizing system is based on Nominal Pipe Size (NPS). The nominal diameter is a reference number that does not match the actual outer diameter, especially in smaller sizes. The wall thickness of a pipe is defined by its schedule, such as Schedule 40 or Schedule 80. A higher schedule number means a thicker wall and therefore a smaller inner diameter for the same nominal size.
For tubes, the sizing convention is more direct. Tubing is typically specified by its actual outer diameter, and the wall thickness is stated separately, usually in millimetres or inches. This means that a tube listed as 50 mm OD with a 3 mm wall thickness actually measures 50 mm across the outside.
This difference matters enormously when sourcing materials. Ordering a pipe using tube dimensions, or vice versa, can result in parts that do not fit. If you are ever unsure which system applies to your application, it is always worth checking the relevant standard or asking your supplier directly.
How do you measure the outer diameter of tubing?
The outer diameter is the most straightforward measurement to take. You measure straight across the widest point of the tube, from one outer edge to the other. The most accurate tool for this is a vernier caliper or digital caliper, which can give you a reading to the nearest tenth of a millimetre or thousandth of an inch.
If a caliper is not available, a flexible tape measure can work for larger diameters. Wrap the tape around the tube to get the circumference, then divide by pi (approximately 3.14159) to calculate the outer diameter. This method is less precise but useful in the field when proper measuring tools are not at hand.
Always measure at multiple points along the tube. Slight ovality or deformation can give a misleading reading if you only measure once. For critical applications, take at least two or three readings and use the average.
How do you calculate the inner diameter and wall thickness?
Once you have the outer diameter, you need two more values to fully describe the tube: the wall thickness and the inner diameter.
Wall thickness is best measured directly using a caliper at the end of the tube, or with an ultrasonic thickness gauge if the tube is already installed and the end is not accessible. Ultrasonic gauges are widely used in maintenance and inspection work for exactly this reason.
Once you have the OD and wall thickness, the inner diameter is straightforward to calculate:
- Take the outer diameter measurement.
- Multiply the wall thickness by two (because the wall exists on both sides).
- Subtract that value from the outer diameter.
For example, a tube with a 60 mm OD and a 4 mm wall thickness has an inner diameter of 52 mm. This inner diameter determines the flow capacity of the tube, which is critical for fluid and gas applications.
What are standard tubing sizes and how are they listed?
Standard steel tube sizes are listed according to recognized international standards, the most common being ASTM (American Society for Testing and Materials), DIN (German Institute for Standardization), and EN (European Norm). Each standard defines a range of outer diameters and corresponding wall thicknesses.
In practice, steel tube sizes are typically listed in catalogues and product sheets with the OD first, followed by the wall thickness. For example: 33.7 x 3.2 mm means an outer diameter of 33.7 mm and a wall thickness of 3.2 mm. Some suppliers also list the nominal bore alongside these figures to help cross-reference with pipe standards.
Common steel tube sizes in industrial and maritime applications range from small-bore tubing used in hydraulic and instrumentation systems all the way up to large structural tubes. At Marine Steel, our range extends to 20 inches, covering the full breadth of what maritime, offshore, and construction projects typically require. You can browse our full range on the Marine Steel products page.
What mistakes should you avoid when identifying tubing size?
Even experienced buyers make errors when identifying tubing size. Here are the most common ones worth knowing:
- Confusing nominal size with actual size. A 2-inch pipe does not have a 2-inch outer diameter. Always verify the actual OD before ordering.
- Mixing up pipe and tube standards. Applying pipe schedule logic to tube dimensions, or the other way around, leads to mismatches in fittings and connections.
- Measuring only once. Tubes can be slightly oval, especially after handling or deformation. Take multiple readings.
- Ignoring wall thickness. Two tubes can have the same OD but very different inner diameters if the wall thickness differs. This affects both flow and pressure rating.
- Not referencing the correct standard. ASTM and DIN tubes with similar nominal labels can have different actual dimensions. Always confirm which standard applies to your application.
How Marine Steel helps you get the right tubing size
Getting tubing dimensions right is not always easy, especially when you are working under time pressure or dealing with specifications that are not entirely clear. That is exactly where we come in.
At Marine Steel, we offer more than just stock. We think along with you to make sure you get what you actually need:
- Extensive stock of steel tube sizes across a wide range of ODs and wall thicknesses, up to 20 inches
- Tube processing services including precision cutting to size, bending, drilling, and rolling to your specifications or technical drawings
- Hydraulic tube and fittings supply in both steel and stainless steel for complete system compatibility
- Class certification and custom sizing for projects with specific regulatory requirements
- Locations in Rotterdam and Houston for fast turnaround in key maritime and industrial hubs
- 15+ years of hands-on experience advising clients across maritime, offshore, construction, and industrial sectors
You explain what you need once, and we take it from there. No chasing multiple suppliers, no delays caused by specification errors. If you are ready to source the right tubing for your project, contact us directly and we will get back to you with a fast, accurate quotation.
