If you have ever held a piece of steel tubing and wondered exactly what the stated size refers to, you are not alone. Steel tube sizing is one of those topics that trips up even experienced buyers, and getting it wrong can mean ordering the wrong part, causing delays, or creating a safety issue in a critical system. Whether you are sourcing material for a vessel, an offshore platform, or an industrial project, understanding how steel tubing is measured will save you time, money, and frustration.
Is steel tubing measured by inside or outside diameter?
Steel tubing is most commonly measured by its outside diameter (OD). This is the standard across the vast majority of structural and mechanical tube applications. When a supplier lists a tube as 50mm or 2 inches, that figure refers to the outer edge of the tube wall, not the hollow centre.
This is an important distinction because it directly affects how tubes fit together, how they connect to fittings, and how they perform under pressure or load. The inside diameter (ID) varies depending on wall thickness, so two tubes with the same outside diameter can have very different internal bore sizes.
There is one notable exception worth knowing: steel pipe, which is a related but distinct product, uses a different sizing convention. More on that below.
What’s the difference between steel tube and steel pipe dimensions?
This is where a lot of confusion starts. Steel tube and steel pipe look similar, but their sizing systems are different, and mixing them up can lead to mismatched connections and failed installations.
Steel tube is sized by actual outside diameter. A 60mm steel tube genuinely measures 60mm across the outside. Simple and literal.
Steel pipe uses a nominal bore (NB) or nominal pipe size (NPS) system. These are reference numbers that do not directly correspond to any actual measurement on the pipe. For example, a 2-inch nominal pipe does not measure 2 inches on the outside or the inside. The nominal size is a historical reference that stuck around from older manufacturing conventions.
This matters enormously when you are ordering fittings, flanges, or connecting components. A fitting designed for a 2-inch pipe will not necessarily fit a 2-inch tube. If you are ever unsure which system applies to your application, it is worth speaking with a specialist who can help you clarify the specification before you order.
Why does wall thickness matter when measuring steel tubing?
Once you know the outside diameter, wall thickness becomes the second critical measurement. Together, OD and wall thickness define the tube completely and determine its inside diameter using a straightforward calculation:
Inside diameter = Outside diameter minus (2 x wall thickness)
Wall thickness affects several key performance characteristics:
- Pressure rating: Thicker walls handle higher internal pressures, which is critical in hydraulic and piping systems.
- Load-bearing capacity: Structural applications depend on wall thickness to resist bending and compression.
- Weight: Heavier walls add weight, which matters in weight-sensitive applications like offshore structures or vessel outfitting.
- Weldability and machinability: Certain fabrication processes require minimum wall thicknesses to be carried out safely and accurately.
For pipe products, wall thickness is often expressed as a schedule number (such as Schedule 40 or Schedule 80). Schedule 40 is a standard wall thickness, while Schedule 80 has a thicker wall and is used where higher pressure resistance is needed. The schedule system applies to pipe, not tube, but it is worth understanding if your project involves both.
How do you accurately measure steel tubing on-site?
Measuring steel tubing accurately in the field requires the right tools and a clear understanding of what you are measuring. Here is a reliable approach:
- Use a vernier caliper or digital caliper to measure the outside diameter. Place the jaws against the outer surface at a right angle to the tube axis. Take measurements at two or three points around the circumference to account for any slight ovality.
- Measure wall thickness using an ultrasonic thickness gauge (non-destructive) or a caliper if you have access to a cut end. Measure at multiple points to check for consistency.
- Calculate the inside diameter using the formula above, or measure it directly at the open end of the tube.
- Check the length with a steel tape measure, and note any end conditions such as bevelled ends, threaded ends, or plain cut ends.
When in doubt, always measure rather than assume. Nominal sizes are useful for ordering, but actual dimensions confirm what you have in hand. If you are sourcing replacement material for an existing system, having the actual measured dimensions on hand will help your supplier match the specification precisely.
What are the most common steel tubing size standards?
Steel tubing is produced and sold according to a range of international standards. The most relevant ones for maritime, offshore, and industrial applications include:
ASTM standards are widely used in North American and international markets. ASTM A500 covers cold-formed structural tubing, while ASTM A519 applies to mechanical tubing. These standards define not just dimensions but also material grades, tolerances, and testing requirements. If you are working on a project that requires certified material, the ASTM designation on the mill certificate tells you exactly what you have.
EN standards are common across Europe. EN 10210 and EN 10219 cover hot-finished and cold-formed structural hollow sections respectively, and EN 10305 covers precision tubes for mechanical and hydraulic applications.
Our steel tube and pipe product range covers a broad selection of sizes and standards, including both metric and imperial dimensions, to serve clients across maritime, offshore, construction, and industrial sectors. Tube sizes in our stock run up to 20 inches in outside diameter, giving you access to large-format material without needing to go to a secondary source.
How do you choose the right steel tube size for your application?
Choosing the right tube size comes down to matching the mechanical and dimensional requirements of your application. Start with these questions:
What is the required flow or bore size? If fluid or gas needs to pass through the tube, the inside diameter determines flow capacity. Work backwards from the required ID to find a tube with the right OD and wall thickness combination.
What loads will the tube carry? Structural applications require wall thickness and material grade to be matched to the expected bending, compressive, or tensile forces. An engineer or experienced supplier can help you verify this.
What connections are involved? If the tube needs to connect to fittings, flanges, or other tubes, the OD must match the connection system. This is where the tube versus pipe distinction becomes critical. Mixing sizing systems is a common source of errors that can cause costly rework or, in a worst case, a failed connection under pressure.
What standard or certification is required? Some projects, particularly in offshore and maritime sectors, require material supplied to specific ASTM or EN standards with full documentation. Always confirm the certification requirements before ordering, and make sure your supplier can provide the relevant mill certificates.
If you are working under time pressure and need to get the specification right the first time, speaking directly with a knowledgeable supplier is often the fastest route to the correct answer. At Marine Steel, clients tell us they value being able to explain their situation once and get a clear, practical recommendation in return.
How Marine Steel helps you get the right tube specification, fast
Getting steel tube sizes right matters, and we make that process straightforward. Whether you know exactly what you need or are still working out the specification, we are here to help you move quickly and confidently.
- Broad stock of steel tubes from small-bore to 20-inch OD, in multiple standards and grades
- Tube processing services including precision cutting to size, bending, drilling, and rolling to your specifications or technical drawings
- Full material certification and documentation, including ASTM-certified material for maritime and offshore applications
- One-stop supply covering tubes, fittings, flanges, and related components so you do not need to coordinate multiple suppliers
- Locations in Rotterdam and Houston for fast access to stock across key maritime and industrial hubs
If you are ready to discuss your requirements or want a fast quotation, get in touch with our team and we will help you find the right solution without delay.
