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In the world of pipeline engineering and construction, accurately identifying pipe fitting sizes is crucial for design, procurement, installation, and maintenance. However, the methods used to denote pipe diameters can vary widely depending on material type, regional standards, and industry conventions. Common pipe marking systems include DN (Diameter Nominal), inch-based sizing, D, Φ (phi), De, dn, and d.
DN stands for "Diameter Nominal" or "Nominal Diameter." It represents a standardized nominal value that approximates the internal diameter but does not directly reflect either the actual inner or outer diameter.
DN (Nominal Diameter): Indicates the standardized diameter of a pipe — neither the outer nor inner diameter — but rather a nominal size close to the actual dimensions. For instance, a DN100 pipe typically corresponds to an outer diameter of 108 mm.
This method uses inches as a unit of measurement and is most commonly found in countries following imperial standards or projects involving imported equipment.
Inch: An imperial unit often used to denote nominal diameters. One inch equals 25.4 mm; hence 1 inch (1") corresponds to DN25.
The letter "D" typically refers to the actual external diameter of a pipe and is usually accompanied by wall thickness.
D generally denotes the external diameter of a pipe and is often paired with wall thickness — for example, D108x4 indicates an outer diameter of 108 mm with a wall thickness of 4 mm.
The Greek letter Φ serves a similar role as "D," representing external diameter in engineering drawings.
Φ is commonly used in engineering to indicate circular diameters and functions similarly to "D" in pipe markings — for example, Φ108x4 means an outer diameter of 108 mm with a wall thickness of 4 mm.
De specifies the nominal external diameter primarily used for pipe fitting in building drainage systems.
De (External Diameter): Mainly used for pipe fitting in building water supply and drainage systems; for example, De25x3 indicates an outer diameter of 25 mm with a wall thickness of 3 mm.
The lowercase "dn" refers to pipe fitting as well but aligns more closely with European or ISO standards.
dn usually represents the nominal outer diameter in pipe fitting under European or ISO standards — e.g., dn50 denotes a nominal outer diameter of 50 mm.
"d" typically refers to internal diameters in materials like concrete or clay pipes where flow capacity depends on inner dimensions.
d usually indicates the internal diameter for materials such as concrete or clay pipes — for instance, d300 signifies an internal diameter of 300 mm.
One key distinction lies between nominal dimensions like DN or inch-based markings versus actual measurements like D, Φ, De, dn, or d.
· DN is widely adopted across Chinese and international standards for various materials including steel, cast iron, and plastics in water supply, HVAC, gas pipelines etc.
· D and Φ are common in metal piping standards domestically and internationally where dimensional accuracy matters such as mechanical manufacturing or petrochemical sectors.
· d is mainly used for traditional materials where inner diameter defines design parameters like concrete or clay pipes.
· De and dn are prevalent in plastic piping standards; China prefers De while Europe leans toward dn based on standard systems applied.
· Inch-based sizing is mainly seen in imperial-standard countries or international projects using imported equipment.
Qingdao Fluido provides pipes marked with different sizing conventions such as DN100/D108/Φ108/De110/dn110/d100/inch sizes—accuracy matters. As a leading manufacturer and exporter specializing in steel valves, fittings, and flanges, FLUIDO has been serving over 35 countries/regions for more than two decades.
Whether you require high-pressure forged fittings marked by precise D values or using De/dn specifications—FLUIDO delivers quality you can trust backed by CE certifications and SGS audits.Contact FLUIDO now.
