If you have ever ordered steel for a maritime or industrial project, you have probably come across both the terms “steel pipe” and “steel tubing” and wondered whether they are actually the same thing. They are not. While they look similar and are sometimes confused with one another, pipe and tube are distinct products with different measurement systems, different standards, and different applications. Choosing the wrong one can cause real problems on the job, from failed pressure ratings to incompatible fittings. This article breaks down the key differences clearly, so you can source with confidence.
How are steel pipe and steel tubing measured differently?
This is where most of the confusion starts. Steel pipe is measured by nominal pipe size (NPS), which is a standardized system that refers to an approximate inside diameter. The outside diameter of a pipe is fixed for a given nominal size, while the wall thickness varies depending on the schedule (for example, schedule 40 or schedule 80). A schedule 80 pipe has a thicker wall than a schedule 40 pipe of the same nominal size, which means the inside diameter is smaller but the outside diameter stays the same.
Steel tubing, on the other hand, is measured by its actual outside diameter and actual wall thickness. What you order is exactly what you get. A 50 mm tube has a true outside diameter of 50 mm. This precision makes tubing the preferred choice in applications where exact dimensions matter, such as mechanical assemblies or structural frameworks.
Understanding this difference is essential when selecting fittings, flanges, or connectors, because a pipe fitting is designed around nominal sizes while a tube fitting is designed around actual dimensions. Mixing the two systems without checking the specifications is a common and costly mistake.
What are the most common uses of steel pipe vs. steel tubing?
Steel pipe is primarily designed to transport fluids and gases under pressure. Its standardized sizing system makes it easy to connect across long runs using compatible fittings, flanges, and valves. Common applications include:
- Water and gas distribution systems on vessels and offshore platforms
- Hydraulic and pneumatic systems in industrial plants
- Process piping in refineries and chemical facilities
- Fire suppression and cooling systems on ships
Steel tubing is more commonly used in structural and mechanical applications where dimensional precision and clean aesthetics matter more than fluid transport. You will find it in machine frames, handrails, boiler tubes, heat exchangers, and structural supports. In the maritime world, tubing appears in everything from deck structures to custom fabrications on vessels.
That said, the line between the two is not always rigid in practice. Some tubing is used in low-pressure fluid applications, and some pipe is used structurally. The key is understanding what your application actually demands before you specify the product.
Which steel grades and standards apply to pipe vs. tubing?
Both pipe and tubing are covered by ASTM standards, but the specific standards differ. For pipe, the most commonly referenced standards include ASTM A53 (general-purpose carbon steel pipe), ASTM A106 (seamless pipe for high-temperature service), and ASTM A312 (stainless steel pipe). These are the grades you will encounter most often in maritime and industrial piping systems.
For tubing, relevant standards include ASTM A513 (mechanical tubing), ASTM A500 (structural tubing), and ASTM A269 (stainless steel tubing for general service). Each standard defines specific requirements for chemical composition, mechanical properties, and testing.
When sourcing either product for critical applications, always verify that the material comes with the appropriate material certificates and documentation. In offshore and maritime environments especially, the wrong grade or an uncertified product can create serious compliance and safety issues.
Can steel pipe and steel tubing be used interchangeably?
In short, no, not without careful verification. Because the two products use different measurement systems and are manufactured to different standards, substituting one for the other without checking the specifications is risky.
The most common scenario where this causes problems is with fittings. Pipe fittings are designed around nominal pipe sizes, and tube fittings are designed around actual outside diameters. Even if the numbers look similar, the tolerances and sealing surfaces are different. Forcing a pipe fitting onto a tube, or vice versa, can result in leaks or mechanical failure.
There are some situations where a tube can perform a structural function that pipe would also serve, particularly when pressure is not a factor. But even then, the wall thickness, grade, and dimensional tolerances need to be checked carefully. When in doubt, always specify the product by its intended standard rather than assuming one can replace the other.
How do you choose the right product for your application?
Start with the function. Ask yourself whether the product needs to carry fluid or gas under pressure, or whether it is primarily structural or mechanical. That single question will point you toward pipe or tube in most cases.
From there, work through the following steps:
- Define the service conditions. What pressure, temperature, and medium will the product be exposed to? This determines the required grade and wall thickness.
- Check the applicable standard. Confirm which ASTM or other standard applies to your application and whether the product needs to be certified to that standard.
- Specify dimensions correctly. For pipe, use nominal pipe size and schedule. For tubing, specify actual outside diameter and wall thickness. Getting this right prevents ordering errors.
- Consider the fittings and connections. Make sure the fittings, flanges, and connectors you plan to use are compatible with the product type and size you have specified.
- Think about lead time and availability. In time-sensitive situations, the product that is in stock and ready to ship is often the right choice, as long as it meets the technical requirements.
If you are working with steel tube sizes and are unsure which schedule, grade, or standard applies to your project, it is always better to ask before ordering. A good supplier will help you work through the specification rather than just fulfilling an order blindly.
How Marine Steel helps you source the right product
We stock an extensive range of steel pipes and tubes across multiple grades, sizes, and standards, all from our warehouses in Rotterdam and Houston. Whether you need certified pipe for a pressure system or precision tubing for a structural application, we can help you get it right the first time.
Here is what we bring to every order:
- Broad stock of steel pipes and tubes, including schedule 40 and schedule 80, up to 20 inches
- ASTM-certified products with full material documentation available
- Tube processing services including cutting to size, bending, drilling, and rolling to your exact specifications
- Hydraulic tube and fittings in steel and stainless steel for complete system compatibility
- Expert advice on grade selection, sizing, and standards, so you do not have to figure it out alone
- One-stop sourcing from a single reliable supplier with 15 years of maritime and industrial experience
Not sure whether you need pipe or tube for your next project? Get in touch with our team and we will help you find the right product fast.
