Buying steel tube sounds straightforward until you are standing in front of a product catalogue with dozens of sizes, wall thicknesses, and standards listed side by side. If you have ever asked yourself “what size tube do I actually need?”, you are not alone. Whether you are sourcing for a vessel, an offshore platform, or an industrial project, getting the tube size right from the start saves you time, money, and a lot of back-and-forth. This guide walks you through everything you need to know about steel tube sizes in plain language.
What does tube size actually mean?
When people talk about tube size, they are usually referring to two measurements working together: the outer diameter (OD) and the wall thickness. The outer diameter tells you how wide the tube is on the outside, while the wall thickness tells you how much steel sits between the outer surface and the hollow centre.
From those two numbers, you can calculate the inner diameter (ID) by subtracting twice the wall thickness from the outer diameter. The inner diameter matters most when something needs to flow through the tube or when another component needs to fit inside it.
Wall thickness is often expressed as a gauge number or, in industrial and maritime contexts, as a schedule number such as Schedule 40 or Schedule 80. A higher schedule means a thicker wall and a higher pressure rating. Schedule 80, for example, has a significantly thicker wall than Schedule 40 of the same outer diameter, making it suitable for higher-pressure applications.
What’s the difference between tube and pipe sizing?
This is one of the most common sources of confusion, and it is worth clearing up before you place any order.
Tube is sized by its actual outer diameter. When you order a 50 mm tube, the outside of the tube measures 50 mm. Simple.
Pipe is sized by its nominal bore (NB) or nominal pipe size (NPS), which is a reference number that does not directly correspond to any physical measurement. A 2-inch nominal pipe, for instance, does not have a 2-inch outer diameter. The nominal size is a legacy designation that has stuck around because it works as a consistent reference system across fittings, flanges, and valves.
This distinction matters enormously in practice. If you order a tube where you need a pipe, or vice versa, the dimensions will not match and your fittings will not connect. Always confirm with your supplier whether you need tube sizing (actual OD) or pipe sizing (nominal bore) before ordering.
How do you measure a tube correctly?
If you already have a tube in hand and need to match it or replace it, here is how to measure it accurately.
- Outer diameter: Use a caliper or wrap a flexible tape around the outside of the tube to get the circumference, then divide by 3.14159. A caliper is more accurate.
- Wall thickness: Use an ultrasonic thickness gauge if the tube is installed, or a caliper on the cut end if you have access to it.
- Length: Measure the full length you need, including any allowance for connections or fittings at each end.
- Inner diameter: Calculate it from OD minus twice the wall thickness, or measure directly with an inside caliper.
Write all three values down before you call a supplier. The more precise your measurements, the faster and more accurately we can help you find the right match from stock.
What tube size do I need for my application?
The right size depends on what the tube needs to do. A few key questions will guide you in the right direction.
What is flowing through it, and at what pressure? Higher pressure means you need a thicker wall. Hydraulic lines, for example, require much heavier wall specifications than ventilation ducting. If you are working with steam, seawater, or hydraulic fluid, pressure ratings and material compatibility both come into play.
Does something need to fit inside or connect to the outside? If the tube connects to a fitting, flange, or valve, the sizing system must match. A tube connected to a standard pipe fitting needs to follow pipe sizing conventions, not tube sizing.
What loads will the tube carry? Structural tubes used in frames, supports, or offshore equipment need to be sized for mechanical strength, not just flow. Wall thickness and steel grade both contribute to load-bearing capacity.
When in doubt, describe the application rather than guessing a size. A knowledgeable supplier can work backwards from what you need the tube to do and recommend the correct specification. You can find an overview of available products on our marine steel products page.
What are the most common tube sizes in maritime and industrial use?
While every project is different, certain steel tube sizes appear consistently across maritime, offshore, and industrial applications. Outer diameters ranging from 21.3 mm (half inch nominal) up to 508 mm (20 inch) cover the vast majority of use cases. Our stock runs up to 20 inch, which handles everything from small hydraulic lines to large structural sections.
In terms of schedule, Schedule 40 is the standard workhorse for moderate-pressure applications such as water lines, general piping, and structural uses. Schedule 80 steps in for higher-pressure systems, chemical lines, and applications where extra mechanical strength is needed. Both are available to ASTM standards, which provide internationally recognised specifications for material composition, dimensions, and mechanical properties.
For maritime and offshore work specifically, stainless steel tubes are frequently specified where corrosion resistance is critical. Carbon steel tubes remain the most common choice for structural and general industrial applications due to their strength and cost-effectiveness.
How do I make sure I order the right tube the first time?
A few straightforward steps will protect you from ordering errors that cause delays and extra costs.
- Confirm whether you need tube or pipe sizing before you start looking at dimensions.
- Write down OD, wall thickness, and length as your three core measurements.
- Identify the applicable standard if your project requires one, such as ASTM A106 for seamless carbon steel pipe or ASTM A53 for general-purpose pipe. Certifications and material documentation may be required, especially for offshore and marine class-approved projects.
- Check the end connections to confirm that the tube size matches whatever it will connect to.
- Describe the application to your supplier if you are unsure. A good supplier will ask the right questions and help you arrive at the correct specification.
Time pressure is real in maritime and industrial procurement. The last thing you want is a wrong delivery holding up a vessel or a construction project. Getting the specification right before you order is always faster than dealing with returns and reorders after the fact.
How Marine Steel helps you get the right tube size
We understand that not every buyer arrives with a complete specification sheet. That is exactly why we take a hands-on approach: you tell us what you need the tube to do, and we help you figure out the right size, grade, and standard.
- Extensive stock of steel tubes from small bore up to 20 inch, in carbon steel and stainless steel
- ASTM-certified material with full documentation available on request
- Custom cutting, bending, and processing to your exact dimensions
- Hydraulic tube and fittings in steel and stainless steel for complete system compatibility
- Class certification available for maritime and offshore applications
- Warehouses in Rotterdam and Houston for fast turnaround wherever you are based
Whether you know exactly what you need or you are still working it out, we are here to help you get it right the first time. Contact us and tell us about your application. We will come back to you quickly with the right product and the right advice.
