The nominal diameter is the size of all the piping accessories in the piping system. It is a convenient round integer for reference, and has no strict relationship with the machining size. The letter “DN” represents for the nominal diameter and followes a numerical symbol.

What is the nominal diameter?

The mean outside diameter is also meaning the nominal diameter. This is the edge from the metal pipe wall is very thin. The outer diameter and the inner diameter of the tube is almost the same. So take the average of the outside diameter and inside diameter as a pipe diameter. DN is the common diameter of various pipes and pipe accessories. It is same for the pipe and pipe accessories connect with each other, with interchangeability. It is not the actual sense of the outside diameter or inside diameter of pipe, although its value with the pipe diameter is closer or equal. In order to make the pipe and pipe fitting uniform, we will use nominal diameter.
For example, welded steel pipe can be divided into thin-walled steel pipe, ordinary steel pipe and thickened steel pipe. Its value is not the outside diameter, nor the inside diameter, but a nominal size of approximate ordinary steel pipe inside diameter. Each nominal size corresponds to an outer, and the inner value varies with the thickness. It uses metric mm or in inch to express. Nominal diameters also use for pipe accessories, meaning the same as the seam pipe.

Symbol Interpretation

Generally speaking, the diameter of the tube can be divided into outside diameter, inside diameter, nominal diameter. The letter D represents the outer diameter of the seamless steel pipe. Followed by the size and wall thickness of the additional outer diameter. For example, the outside diameter of 108 seamless steel pipe, wall thickness of 5MM, with D108 * 5 said, plastic pipe also with outside diameter, such as De63, other such as reinforced concrete pipe, cast iron pipe, galvanized steel pipe using DN said. It is generally to express in the design drawings. It is useful to design, manufacture and maintenance of convenient artificial provisions of a standard. Also more than the nominal size, is the pipe (or pipe) specification name.

The nominal size of the pipe and its internal diameter, outside diameter are not equal. For example: the nominal size of 100MM seamless steel pipe 102 * 5, 108 * 5 and several other kinds, 108 for the pipe outside diameter. Therefore, the inner diameter of the steel tube is (108-5-5) = 98mm. But it is not exactly equal to the difference between the outer diameter of steel tube minus two times the wall thickness.

In other words, we can say the nominal size is close to the inside size. But is not equal to the diameter of a pipe diameter specification name. In the design drawings use a nominal size, the purpose is to determine the size of the pipe, pipe fittings, valves, flanges, gaskets and other structural dimensions and connecting dimensions. In the design drawing, a table of pipe specifications will use to indicate a pipe’s nominal diameter and wall thickness.

Size Conversion:

Converted to:
1 inch = 2.54cm = 25.4mm
1 foot = 12 inches = 30.48cm = 304.8mm

Pipe Nominal Diameter