Is Nominal Diameter (DN) The Outer Diameter Or Inner Diameter?

In the selection and installation of industrial pipelines, valves, flanges, and other equipment, "Nominal Diameter (DN)" is one of the most frequently used technical terms. However, many new entrants to the industrial field and even some senior technical personnel often fall into a cognitive misunderstanding: equating nominal diameter with the Outer Diameter (OD) or Inner Diameter (ID) of pipelines or valves.

In fact, nominal diameter is a "nominal size" that neither directly corresponds to the outer diameter nor exactly equals the inner diameter. It is a unified benchmark established by the industrial sector to achieve equipment standardization and interchangeability.

This article will explain this core technical concept in plain language with real cases from four dimensions: the essential definition of nominal diameter, the key differences from outer diameter/inner diameter, the application logic under different standards, and common misunderstandings in practical selection, helping industry practitioners accurately grasp its application principles.

According to the definition in GB/T 1047 "Nominal Diameter of Pipe Components", nominal diameter is "an identifier consisting of a combination of letters and numbers for pipe system components, which is composed of the letter DN followed by a dimensionless number".

The key here is the "dimensionless number" — that is, the value after DN does not directly correspond to any actual dimension of the pipeline or valve (neither the outer diameter nor the inner diameter), but a "code" used to identify the specification.

For example, a DN50 pipeline may have an outer diameter of 57mm (welded steel pipe), 63mm (PE pipe), or 50.8mm (imperial steel pipe), and the inner diameter varies with the wall thickness. However, all pipeline fittings (valves, flanges, joints, etc.) marked DN50 can be mutually matched.

This is similar to the "XL size" in clothing — the actual size of XL may vary among different brands, but it all represents "extra large size", allowing consumers to choose the appropriate size based on this unified identifier. The core function of nominal diameter is to establish a cross-manufacturer and cross-material "unified language" for industrial equipment.

In the early stage of the Industrial Revolution, the production of pipelines and valves lacked unified standards. Products of the same specification produced by different manufacturers had significant dimensional differences, resulting in inability to interchange equipment and bringing great troubles to engineering installation and maintenance. For instance, a "2-inch valve" produced by one manufacturer might have an inner diameter of 50mm, while a "2-inch valve" from another manufacturer might have an inner diameter of 52mm, making these two valves incompatible with the same section of pipeline.

To address this issue, countries around the world have successively formulated nominal diameter standards, classifying pipelines and fittings of different materials and wall thicknesses into a unified nominal diameter series according to functional requirements.

For example, pipelines transporting media such as water, steam, and gas are set with specifications like DN15, DN20, DN25... based on flow rate requirements. Regardless of the material (carbon steel, stainless steel, or plastic), as long as the nominal diameter is the same, their connection dimensions (flange hole diameter, thread specification, etc.) remain consistent, thereby realizing standardized production and interchangeable installation.

The essential function of nominal diameter is to "locate connection dimensions" rather than describe the internal flow space of the pipeline. The connection reliability of industrial equipment depends on "mating dimensions" (such as the center distance of flange bolt holes, the outer diameter of valve interfaces, the thread profile specification, etc.), which are uniquely determined by the nominal diameter.

Taking flanges as an example, for a DN100 flange, key connection dimensions such as the bolt hole center circle diameter, number of bolts, and bolt specification are fixed (e.g., GB/T 9119-2010 stipulates that for a DN100, PN1.6 flange, the bolt hole center circle diameter is 150mm, the number of bolts is 8, and the bolt specification is M16). Whether the supporting pipeline is thick-walled or thin-walled, as long as the nominal diameter is DN100, it can perfectly match this flange.

This is the core value of nominal diameter — it eliminates the impact of variables such as material and wall thickness on connection dimensions, providing a stable reference coordinate for industrial systems.

The fundamental reason why many practitioners confuse nominal diameter with outer diameter and inner diameter is the lack of clear understanding of their functional positioning: nominal diameter is used to "identify specifications and unify connection dimensions", while outer diameter and inner diameter are used to "describe actual structural dimensions" — they are completely different concepts. The following table and examples detail their differences: