DN40 Pipe Size (48.3mm OD) in mm and Inches

DN40 carbon steel pipe is a steel pipe with a specific outer diameter of 48.3 mm. In practical use, DN40 refers to a nominal diameter rather than an exact internal size, and it corresponds to NPS 1 1/2 inch in international pipe standards.

This size is widely used in fluid transportation systems such as water, gas, oil, and general industrial piping, where a balance between flow capacity and installation space is required. Due to the material properties of carbon steel, DN40 pipes offer reliable strength, good weldability, and stable performance under typical pressure and temperature conditions.

1. DN40 Pipe Size and Dimensions

DN40 is a standard nominal pipe size defined in metric systems, and it corresponds to specific outer diameter values used across international standards.

In most piping standards, the outer diameter of DN40 remains constant at 48.3 mm, while the wall thickness may vary depending on the schedule (SCH) or standard specification. This allows DN40 pipes to meet different pressure and application requirements without changing the external size.

The wall thickness of DN40 pipes varies by schedule (SCH), which directly affects the inner diameter (ID), weight, and pressure capacity of the pipe.

DN40 Pipe Wall Thickness and Schedule (SCH 10, 40, 80)

As the schedule increases, the wall thickness becomes larger, resulting in a smaller inner diameter and higher pressure resistance.

2. DN40 Carbon Steel Pipe Specifications

DN40 carbon steel pipes are commonly manufactured according to several international standards, depending on the application and performance requirements.

ASTM A53 – Typically used for low to medium pressure applications such as water, gas, and general structural use. DN40 pipes under this standard are available as both seamless steel pipe and ERW pipe, depending on manufacturing method and application requirements.

ASTM A106 – Designed for high-temperature and high-pressure service, commonly used in oil, gas, and power plant systems.

EN 10255 – A European standard for non-alloy steel pipes suitable for welding and threading, widely used in building services and mechanical applications.

Although the outer diameter remains the same, these standards differ in terms of mechanical properties, chemical composition, and allowable pressure ranges. Selecting the correct specification depends on the working conditions, including temperature, pressure, and corrosion environment.

3. Applications of DN40 Carbon Steel Pipe

DN40 carbon steel pipe is typically selected in systems where moderate flow capacity is required without increasing installation space or cost. In practice, it sits in a very “middle” position—larger than small branch lines, but not yet part of main transmission pipelines.

It is commonly used in:

Water supply systems – for building distribution lines, pump connections, and general service piping where stable pressure and durability are needed.

Gas pipelines – especially in low to medium pressure transmission, where consistent diameter and reliable sealing are important.

Oil and industrial piping – including process lines, machinery connections, and auxiliary systems in factories and plants.

In many real projects, DN40 is chosen not because it is “standard,” but because it offers a practical balance: sufficient flow capacity without the material cost and space requirements of larger diameters like DN50 or DN65.

4. Advantages of DN40 Carbon Steel Pipe

Compared with other materials or pipe sizes, DN40 carbon steel pipe provides a combination of mechanical strength, workability, and installation flexibility that suits a wide range of applications.

4.1 High strength and toughness

Carbon steel provides sufficient strength to withstand pressure and temperature variations in typical piping systems, while maintaining structural stability under load.

4.2 Good plasticity and weldability

DN40 pipes can be easily processed, bent, and welded using common methods such as manual arc welding or gas shielded welding, making them suitable for complex layouts.

4.3 Flexible connection methods

Welding, threaded, and flange connections are all applicable, allowing adaptation to both permanent installations and systems requiring maintenance or disassembly.

4.4 Adaptability to surface treatment

Although carbon steel itself is not corrosion-resistant, DN40 pipes can be protected through coating, painting, or galvanizing to extend service life in different environments.

5. DN40 vs DN50: What’s the Difference?

DN40 and DN50 are both commonly used nominal pipe sizes, but they serve different roles in piping systems depending on flow requirements and system design.

"People often use DN40 for branch lines or secondary runs, and DN50 where you want more flow or lower pressure loss. The choice isn't personal preference — it's all about the system: flow, pressure, and installation space.

6. Connection Methods for DN40 Carbon Steel Pipe

DN40 carbon steel pipe supports several commonly used connection methods, and the choice usually depends on installation conditions, maintenance requirements, and system pressure.

6.1 Welding connection

Welding is the most widely used method for DN40 pipes, especially in permanent piping systems. It provides strong joint strength and good sealing performance, making it suitable for applications where leakage control and structural integrity are critical.

6.2 Threaded connection

Threaded connections are typically used in smaller-scale systems or where frequent disassembly is required. For DN40 pipes, threading is still practical and allows for easier installation without specialized welding equipment.

6.3 Flange connection

Flange connections are commonly applied in systems that require regular maintenance, inspection, or connection to equipment such as valves and pumps. They offer convenience in assembly and disassembly, especially in more complex piping layouts.

7. FAQs

Q1: What is DN40 pipe size in mm and inches?

DN40 pipe has an outer diameter of 48.3 mm and corresponds to NPS 1 1/2 inch.

Q2: Is DN40 the same as 1 1/2 inch pipe?

Yes, DN40 is equivalent to NPS 1 1/2 inch in international pipe sizing systems, although DN refers to nominal diameter rather than exact internal size.

Q3: What is DN40 pipe used for?

DN40 pipes are commonly used in water supply systems, gas pipelines, and general industrial piping where moderate flow capacity is required.

Q4: Can DN40 be used for high pressure applications?

It depends on the material standard and wall thickness. For example, ASTM A106 pipes with appropriate schedule can be used in higher pressure and temperature conditions.

8. Conclusion

DN40 carbon steel pipe is widely used in engineering projects. It typically covers medium-flow applications, sitting between small branch lines and larger DN50 pipelines. Because of this position, it is commonly found in distribution systems, equipment connections, and general industrial piping networks.

At the project planning stage, it is important to confirm specifications early. Choosing between seamless and welded pipes, as well as matching the correct pressure class and standard, helps avoid adjustments during installation and reduces on-site rework.