Pipe Fittings Sizing Chart: DN to NPS Conversion for Industrial Piping

In the world of industrial piping, where precision can mean the difference between a smoothly running operation and a costly shutdown, few details matter more than pipe sizing. Whether you're overseeing pipeline works in a petrochemical facility, installing pressure tubes in a power plant, or fitting components for marine ship-building, getting the dimensions right is non-negotiable. Yet, for many professionals, the confusion between DN (Nominal Diameter) and NPS (Nominal Pipe Size) can feel like navigating a maze without a map. These two systems—one rooted in metric conventions, the other in imperial—govern how pipes, flanges, valves, and fittings are specified and sourced globally. Misinterpreting them can lead to mismatched components, leaks, or even system failures. That's why we've put together this guide: to demystify DN and NPS, provide a clear conversion chart, and explain why this knowledge is the backbone of successful industrial projects.

What Are DN and NPS, and Why Do They Matter?

Let's start with the basics. DN, short for "Nominal Diameter," is a metric standard defined by ISO (International Organization for Standardization) and widely used in Europe, Asia, and other metric-focused regions. It's a numerical value, expressed in millimeters (e.g., DN15, DN50), that represents a pipe's approximate internal diameter but is not an exact measurement. Think of it as a "name" for the pipe size rather than a precise dimension. For example, a DN100 pipe doesn't have a 100mm internal diameter—its actual dimensions depend on the pipe's wall thickness and material, which vary based on pressure ratings and application.

NPS, or "Nominal Pipe Size," is the imperial counterpart, used primarily in the United States and countries that follow ANSI (American National Standards Institute) standards. Expressed in inches (e.g., NPS ½, NPS 4), it's also a nominal designation, not a direct measurement of the pipe's inside or outside diameter. Historically, NPS originated from the practice of referring to pipe sizes by their approximate internal diameter in inches, but like DN, it has evolved into a standardized label. For instance, an NPS 2 pipe isn't exactly 2 inches inside—it's a classification that aligns with industry-agreed outside diameters and wall thicknesses (schedule numbers like SCH 40 or SCH 80).

The reason these systems exist is simple: consistency. In global supply chains, a DN50 pipe ordered from a manufacturer in Germany should be compatible with a flange specified as NPS 2 from a supplier in the U.S., provided the conversion is done correctly. This compatibility is critical when sourcing components like pipe flanges, industrial valves, or pipe fittings—items that must seal tightly and withstand high pressures in applications ranging from chemical processing to aerospace. Imagine, for example, installing a DN65 valve onto an NPS 2½ pipe: if the sizing is off by even a few millimeters, the connection could leak, compromising system integrity and safety.

Pro Tip: Remember, DN and NPS are "nominal" for a reason. They don't directly correspond to a pipe's actual inside diameter (ID) or outside diameter (OD). Always cross-reference with OD measurements when ordering, especially for custom components like u-bend tubes or finned tubes, where precise fit is critical.

The DN to NPS Conversion Chart: Your Go-To Reference

To bridge the gap between metric and imperial sizing, we've compiled a DN to NPS conversion table that includes common sizes used in industrial applications. This chart pairs DN values with their equivalent NPS, along with typical outside diameters (OD) in both millimeters and inches—since OD is the most consistent dimension across standards and is often used to match fittings like bw fittings (butt-welded) or sw fittings (socket-welded).

DN (Nominal Diameter, mm)	NPS (Nominal Pipe Size, inches)	Typical OD (mm)	Typical OD (inches)	Common Applications
DN10	NPS ⅜	17.1	0.673	Small-diameter pressure tubes, instrumentation lines
DN15	NPS ½	21.3	0.835	Residential/commercial plumbing, low-pressure pipelines
DN20	NPS ¾	26.9	1.059	Industrial machinery, compressed air lines
DN25	NPS 1	33.7	1.327	Process piping, heat exchanger tubes (small scale)
DN32	NPS 1¼	42.4	1.669	Water supply systems, cooling loops
DN40	NPS 1½	48.3	1.902	Chemical processing lines, fire sprinkler systems
DN50	NPS 2	60.3	2.374	Pressure tubes, pipeline works, petrochemical facilities
DN65	NPS 2½	76.1	2.996	Marine ship-building, HVAC systems
DN80	NPS 3	88.9	3.5	Power plant piping, large-scale water distribution
DN100	NPS 4	114.3	4.5	Main pipeline works, industrial valves installations
DN125	NPS 5	139.7	5.5	Oil and gas pipelines, heavy-duty process lines
DN150	NPS 6	168.3	6.625	Petrochemical facilities, steam lines
DN200	NPS 8	219.1	8.625	Large-diameter pipeline works, marine applications
DN250	NPS 10	273.0	10.75	Water treatment plants, industrial process lines
DN300	NPS 12	323.9	12.75	Power plants, aerospace testing facilities

This chart covers the most commonly used sizes, but industrial projects often require larger or specialized dimensions—like DN400 (NPS 16) for high-capacity pipeline works or DN500 (NPS 20) for marine ship-building. For these, always consult the latest standards (e.g., ANSI B36.10 for NPS, ISO 6708 for DN) or work directly with suppliers who offer custom sizing, as wall thicknesses and pressure ratings can further complicate compatibility.

Common Pitfalls in DN/NPS Conversion (and How to Avoid Them)

Even with a conversion chart, missteps can happen. Here are three of the most common mistakes we've seen in the field—and how to steer clear of them:

1. Confusing DN with Actual Internal Diameter

One of the biggest myths is that DN directly equals a pipe's internal diameter (ID). In reality, DN is a rounded metric value that's often close to the ID of older, thin-walled pipes, but modern pipes—especially pressure tubes or those with thick walls (like SCH 80 or SCH 160)—have IDs much smaller than their DN. For example, a DN100 pipe with SCH 160 wall thickness has an ID of only ~87mm, far less than 100mm. Relying on DN alone to estimate flow rates or pressure ｄｒｏｐ can lead to undersized systems. Always calculate ID using the formula: ID = OD - 2*(wall thickness).

2. Overlooking Flange and Fitting Compatibility

Pipe flanges, whether steel flanges, copper nickel flanges, or custom designs, are sized to match the pipe's OD, not DN or NPS. A DN50 (NPS 2) flange, for instance, is designed to fit a pipe with a 60.3mm OD—regardless of the pipe's wall thickness. If you order a flange based solely on DN without confirming OD, you might end up with a part that won't seat properly, leading to leaks. The same goes for pipe fittings like threaded fittings or socket-weld (sw) fittings: their bore size is based on the pipe's OD, not nominal size.

3. Ignoring Regional Standards

While DN and NPS are globally recognized, regional variations can trip up even seasoned professionals. For example, a DN25 pipe in Europe might follow EN standards, while an NPS 1 pipe in the U.S. adheres to ANSI B36.10. These standards can have slight differences in OD tolerances or wall thicknesses, which matter when mixing components from different suppliers. When working on international projects—say, sourcing copper nickel flanges from Asia for a Middle Eastern petrochemical facility—always confirm that the supplier is using the same base standard (e.g., ASME for NPS, ISO for DN) as your project.

Case Study: A recent project in a marine ship-building yard required connecting a DN200 (NPS 8) pipeline from a European supplier with NPS 8 valves from a U.S. manufacturer. The team assumed DN200 and NPS 8 were identical, but the European pipe had an OD of 219.0mm, while the U.S. valves were sized for 219.1mm OD. Though the difference was just 0.1mm, it caused the gaskets to fail during pressure testing. The fix? Specifying OD-matched components upfront. Lesson learned: when in doubt, confirm OD.

Beyond the Chart: Sizing for Specialized Applications

While the DN/NPS conversion chart covers standard pipes, many industrial applications demand specialized components that add layers of complexity. Let's look at a few scenarios where precise sizing is critical:

Heat Efficiency Tubes: U-Bend and Finned Tubes

In power plants and petrochemical facilities, heat exchanger tubes like u-bend tubes or finned tubes are designed to maximize heat transfer. These components often have non-standard dimensions, with finned tubes adding external ridges to boost surface area. When specifying these, you'll need to reference both the base pipe size (e.g., DN25/NPS 1) and the fin dimensions (height, thickness, spacing). For u-bend tubes, the bend radius must also be considered to ensure they fit within the heat exchanger's shell. Suppliers offering custom heat efficiency tubes will typically ask for OD, wall thickness, and bend radius—not just DN or NPS.

High-Pressure and Nuclear Applications

Pressure tubes in nuclear facilities (e.g., RCC-M Section II nuclear tubes) or high-pressure pipelines (like those in oil and gas) have strict sizing requirements to withstand extreme conditions. These often follow specialized standards (e.g., ASTM B163 for nickel alloy tubes, BS2871 for copper alloy tubes) that dictate OD, wall thickness, and tolerance levels far more tightly than general industrial pipes. In these cases, DN and NPS are secondary to the standard's specific dimensions—always reference the material standard first.

Custom Fabrication for Marine and Aerospace

Marine ship-building and aerospace projects frequently require custom-sized pipes and fittings to fit tight spaces or meet unique weight constraints. For example, a custom big diameter steel pipe for a ship's hull might need a non-standard OD to align with structural works, while aerospace applications might use thin-walled, lightweight alloys (like B165 Monel 400 tubes) with precise OD tolerances. In these cases, working with a supplier that offers custom fabrication is key—and providing detailed drawings with OD, ID, and material specs (not just DN/NPS) will ensure the part fits perfectly.

Final Thoughts: Making Sizing Work for You

At the end of the day, DN and NPS are tools to simplify communication in a global industry—but they're not substitutes for precision. Whether you're ordering off-the-shelf pipe flanges for a routine pipeline works project or custom alloy steel tubes for a cutting-edge aerospace application, the golden rule remains: confirm OD, wall thickness, and material standards. Use this conversion chart as a starting point, but always cross-verify with suppliers, reference the latest industry standards, and when in doubt, ask for a sample or technical drawing.

In the world of industrial piping, where every millimeter counts, mastering DN and NPS conversion isn't just a skill—it's a responsibility. It ensures that the pressure tubes in your power plant operate safely, the pipeline works in your petrochemical facility run efficiently, and the marine ship-building project you're overseeing sails smoothly. So keep this guide handy, share it with your team, and let precision be the foundation of your next project.