If you have ever tried to order steel tubing and found yourself staring at a wall of numbers, you are not alone. Outer diameters, wall thicknesses, nominal sizes, schedules — the terminology can feel overwhelming, especially when you are working under time pressure and need the right material fast. This guide breaks down steel tube sizes in plain language, so you can make confident decisions whether you are outfitting a vessel, completing an industrial installation, or managing a construction project.
What is steel tubing and how is it measured?
Steel tubing is a hollow, cylindrical metal product used across a wide range of industries — from shipbuilding and offshore platforms to structural construction and mechanical systems. It carries fluids, gases, and structural loads, and the right size is critical to both safety and performance.
Steel tubing is measured using three key dimensions:
- Outer diameter (OD): the total width of the tube from one outside edge to the other
- Wall thickness: how thick the tube wall is, which directly affects strength and pressure rating
- Inner diameter (ID): the usable internal space, calculated as OD minus twice the wall thickness
These three measurements work together. A tube with a large OD but thin walls behaves very differently from one with the same OD and heavy walls. Understanding this relationship is the foundation of choosing the right steel tube for your application.
What standard sizes does steel tubing come in?
Steel tubing is available in a wide range of standard sizes, and the exact range depends on the product type and applicable standard. For structural and mechanical steel tubes, common outer diameters start at around 10 mm and run through 20, 25, 30, 40, 50, 60, 76, 89, 101, 114, 127, 139, 152, 168, 193, 219, 273, 323, 355, 406, and 457 mm — and beyond for large-diameter industrial applications.
In industries where imperial sizing is standard — particularly in maritime, offshore, and North American industrial contexts — steel tubing is commonly referenced in inches. Sizes typically range from ½ inch up to 20 inches in outer diameter, covering the vast majority of commercial and industrial applications.
For steel pipes and related products that follow ASTM standards, sizing is organised around nominal pipe size (NPS), which is a standardised designation rather than an exact measurement. We will cover the distinction between pipe and tube sizing in the next section.
What’s the difference between steel tube and steel pipe sizes?
This is one of the most common points of confusion in the industry, and it matters in practice.
Steel tube is specified by its actual outer diameter and wall thickness. What you see on the specification sheet is what you get. A 50 mm OD tube with a 3 mm wall has those exact dimensions.
Steel pipe, on the other hand, is specified using nominal pipe size (NPS) — a standardised designation that does not correspond directly to any actual measurement. For example, a 2-inch NPS pipe does not have a 2-inch outer diameter. Its OD is actually 60.3 mm (approximately 2.375 inches). The nominal size is simply a label used across the industry to ensure compatibility between pipes, fittings, flanges, and valves.
This distinction becomes especially important when you are assembling a complete piping system. A mismatch between tube and pipe sizing conventions can lead to serious compatibility issues — which is exactly the kind of problem that costs time and money when a vessel is in port or a platform is mid-installation.
If you are ever unsure which sizing convention applies to your project, our team is happy to help you work through the specifications before you place an order. You can reach us through the Marine Steel contact page.
What wall thickness options are available for steel tubing?
Wall thickness is where much of the technical decision-making happens. Thicker walls mean higher pressure ratings, greater structural strength, and more weight. Thinner walls are lighter and more cost-effective but suited only to lower-pressure or non-structural applications.
For pipes following ASTM standards, wall thickness is expressed using a schedule designation. Common schedules include:
- Schedule 10: thin-walled, used for low-pressure applications such as drainage and ventilation systems
- Schedule 40: the most widely used schedule, suitable for standard pressure applications across maritime, industrial, and construction sectors
- Schedule 80: heavier wall for higher-pressure applications, common in hydraulic and process piping
- Schedule 160 and XXS (double extra strong): used in demanding high-pressure environments such as offshore and subsea applications
For structural steel tubes, wall thickness is typically specified directly in millimetres and ranges from 1.5 mm for light sections up to 20 mm or more for heavy structural applications. The correct choice depends on your load requirements, pressure ratings, and any applicable industry certifications.
Which steel tubing size is right for my application?
The right steel tube size depends on three factors working together: what the tube needs to carry or support, the environment it will operate in, and any standards or certifications that apply to your sector.
For fluid and gas transport, the key variables are the operating pressure, the medium being transported, and the required flow rate. Higher pressures demand thicker walls and appropriate schedule ratings. For structural applications, the governing factors are load-bearing requirements, span length, and whether the tube needs to meet specific structural standards.
In maritime and offshore environments, material grade and certification are equally important alongside size. ASTM standards — such as ASTM A106 for seamless carbon steel pipes or ASTM A53 for welded and seamless pipes — define not just dimensions but also material properties and testing requirements. Getting the grade right is just as important as getting the size right.
If you are working with a technical drawing or specification sheet, the sizing information will usually be stated explicitly. If you are working from a system description or a general requirement, it helps to consult with a supplier who understands the application — not just the catalogue.
How Marine Steel helps you find the right steel tube size
Choosing the correct steel tube size is straightforward when you have the right support behind you. At Marine Steel, we stock an extensive range of steel tubes from ½ inch up to 20 inches, across multiple schedules, grades, and standards — all from our warehouses in Rotterdam and Houston.
- We stock Schedule 40 and Schedule 80 ASTM pipes as standard, with further schedules available on request
- Our team advises on sizing, grade selection, and certification requirements — you explain your application once and we take it from there
- Custom cutting, bending, and processing services are available so your material arrives ready to use
- Material certificates and documentation are provided as standard for traceability and compliance
- We serve maritime, offshore, construction, and industrial clients worldwide as a true one-stop shop
Whether you know exactly what you need or you are still working through the specifications, we are here to help. Get in touch with our team and we will make sure you get the right steel tube size for your project — without delay.
