If you have ever tried to order a steel tube and found yourself staring at a list of numbers that seem to follow no obvious logic, you are not alone. Steel tube sizes can be confusing because the industry uses specific measurement conventions that differ from what you might expect. Whether you are outfitting a vessel, specifying materials for an offshore platform, or sourcing for a construction project, understanding how tube sizes work will save you time, prevent costly mistakes, and help you communicate clearly with your supplier.
This guide walks you through the essentials of steel tube sizes — how they are measured, what sizes are commonly available, and how to choose the right one for your application.
What are the different tube sizes available in steel?
Steel tubes are available in a wide range of sizes to suit virtually every application. Standard sizes typically run from small-diameter tubes of around 6 mm up to large structural tubes of 20 inches (approximately 508 mm) in outer diameter. At Marine Steel, our assortment covers this full range, from precision small-bore tubing to heavy-duty large-diameter pipe.
Common standard outer diameters you will encounter include:
- Small bore: 6 mm, 8 mm, 10 mm, 12 mm, 16 mm, 20 mm
- Medium bore: 25 mm, 32 mm, 38 mm, 42 mm, 48 mm, 60 mm, 76 mm
- Large bore: 88.9 mm, 114.3 mm, 168.3 mm, 219.1 mm, 273 mm, 323.9 mm, up to 508 mm (20 inch)
Wall thickness varies independently of the outer diameter, which means a single outer diameter can come in multiple wall thicknesses. This is why tube specifications always include both the outer diameter and the wall thickness — or, in the pipe world, the schedule number.
What’s the difference between tube size and pipe size?
This is one of the most common points of confusion in the industry, and it is worth getting right before you place an order.
Tube is specified by its actual outer diameter (OD) and wall thickness. What you see on the spec sheet is what you get — a tube listed as 50 mm OD with a 3 mm wall will physically measure 50 mm on the outside.
Pipe, on the other hand, is specified by its nominal pipe size (NPS) — a reference number that does not directly correspond to any physical dimension. A 2-inch NPS pipe does not measure 2 inches on the outside or the inside. Instead, the NPS system was developed to standardise fitting compatibility, so pipes of the same NPS always connect to the same fittings regardless of wall thickness.
Wall thickness for pipes is expressed as a schedule number — for example, Schedule 40 or Schedule 80. A higher schedule means a thicker wall and a higher pressure rating. Schedule 80 is thicker and stronger than Schedule 40 of the same nominal size.
In practice, the terms “tube” and “pipe” are often used interchangeably in conversation, but when specifying materials for a technical project, the distinction matters. If you are unsure which system applies to your application, we are happy to help you work through the specifications.
How are tube dimensions measured and specified?
For steel tube, the key dimensions are:
- Outer diameter (OD) — the total width of the tube measured from the outside edge to the outside edge
- Wall thickness (WT) — the thickness of the tube wall, which determines strength and pressure rating
- Inner diameter (ID) — calculated as OD minus twice the wall thickness; relevant when flow capacity matters
- Length — standard mill lengths are typically 6 metres, but tubes can be cut to size for specific project requirements
For pipes, the specification uses nominal pipe size (NPS) and schedule number, as described above. Material grade is always part of the specification — for example, ASTM A106 Grade B for carbon steel pipes, or ASTM A312 for stainless steel pipes. The full range of steel products we stock covers multiple grades and specifications to suit different pressure, temperature, and corrosion requirements.
When ordering, always provide the outer diameter, wall thickness, material grade, and length. If you are working from a drawing or a class society specification, share that with your supplier — it removes any ambiguity.
What tube sizes are used in maritime and offshore applications?
Maritime and offshore projects typically demand a broad range of tube sizes, often within a single order. A vessel or offshore platform uses tubing across many systems, each with its own size and material requirements.
Smaller diameter tubes — typically in the 10 mm to 60 mm OD range — are common in hydraulic systems, instrumentation lines, and cooling water circuits. Medium-bore tubes from 60 mm to 168 mm are widely used in fire suppression systems, ballast lines, and fuel systems. Larger diameters above 168 mm appear in structural applications, main seawater lines, and cargo handling systems.
Material selection is equally important in these environments. Stainless steel tube is preferred where corrosion resistance is critical. Copper-nickel alloys are frequently specified for seawater service. Carbon steel with appropriate coatings or linings is used where cost efficiency matters and the environment is less aggressive.
Class certifications — from Lloyd’s Register, DNV, Bureau Veritas, and others — are often required for tubes used in safety-critical systems on board vessels. Documentation and material certificates are a standard part of any compliant maritime supply. If you need certified material, make sure to request the relevant certificates when placing your order.
How do you choose the right tube size for your project?
Choosing the right tube size comes down to four key questions:
What is the application? Structural tubing, pressure service, fluid transport, and hydraulic systems all have different requirements. The application defines the material grade, wall thickness, and any certification requirements.
What are the pressure and temperature conditions? Higher pressure and temperature demand thicker walls and specific material grades. Schedule 80 provides more pressure resistance than Schedule 40 of the same nominal size. For high-temperature service, material grade selection becomes critical.
What are the connection requirements? If the tube needs to connect to fittings, flanges, or other components, the size system must be consistent. Mixing tube OD specifications with pipe NPS specifications without checking compatibility is a common source of error.
What certifications are required? Offshore and maritime projects often require specific ASTM grades and class society certification. Knowing this upfront prevents delays caused by sourcing non-compliant material and then having to reorder.
If you are working through a complex specification or are not sure which size is correct, the right move is to contact your supplier early. A knowledgeable supplier will ask the right questions and help you avoid mistakes before they become expensive problems.
How Marine Steel helps you find the right tube size
We stock steel tubes across the full size range — from small-bore precision tube to 20-inch large-diameter pipe — in carbon steel, stainless steel, and non-ferrous materials. Our team understands the technical requirements of maritime, offshore, construction, and industrial projects, and we are set up to help you get the right specification the first time.
Here is what we offer:
- Extensive stock of standard tube sizes available for fast delivery from Rotterdam and Houston
- Custom cutting, bending, drilling, and rolling to your exact dimensions or technical drawings
- Hydraulic tube and fittings in steel and stainless steel for complete system compatibility
- Class certification and material certificates where required
- Conservation treatments, galvanizing, and sand blasting for corrosion protection
- Technical advice on material grade, wall thickness, and specification — no need to figure it out alone
We know that time pressure is real — a vessel waiting in port cannot wait for the wrong material to be reordered. That is why we make it easy to get the right tube, cut to size, with the right documentation, as quickly as possible. Contact us to discuss your requirements and get a fast quote.
