DN, De, and Φ Explained: Understanding the Differences

This article primarily focuses on “pipe diameter,” a rather perplexing subject due to its various representation methods. I’ll explain the usage and differences between outer diameter (De), the diameter/outer diameter of a regular circle (Φ), and nominal diameter (DN).

What do DN, De, and Φ represent?

1. DN – Nominal Diameter of the Pipe

Commonly used to describe: galvanized steel pipes

The nominal diameter can be represented in metric mm or in inches. Piping accessories also use nominal diameter for representation, with the same implications as seam pipes. Its correlation with the imperial system is as follows:

Inches (in)DNNominal Outer Diameter (mm)

Nominal diameter, also known as mean outside diameter, originates from the fact that metal pipes have thin walls, and the difference between the outer diameter and the inner diameter is negligible. Therefore, the average of the outer diameter and the inner diameter of the pipe is used as the nominal diameter.

DN stands for nominal diameter, which is the universal caliber of various pipes and pipe fittings. The nominal diameter is a convenient round integer for reference, which is only loosely related to the processing size. Pipes and pipe fittings with the same nominal diameter can be interchanged. It is not the actual outer or inner diameter of the pipe, although its value is close to or equal to the inner diameter of the pipe.

For example, welded steel pipes can be divided into thin-walled steel pipes, ordinary steel pipes, and thickened steel pipes according to thickness. Its nominal diameter is neither outside diameter nor inside diameter, but a nominal size close to the inner diameter of ordinary steel pipes. Each nominal diameter corresponds to an outside diameter, and its inner diameter varies with different thicknesses.

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2. De – Outer Diameter of the Pipe

De primarily refers to the outer diameter of a pipe. It is mainly used to describe seamless steel pipes, PVC and other plastic pipes, as well as other pipe materials where the wall thickness needs to be clearly stated. Pipes generally marked with De need to be labeled in the format of outer diameter X wall thickness.

Take the galvanized welded steel pipe as an example; the labeling methods using DN and De are as follows:

  • DN20 De25X2.5mm
  • DN25 De32X3mm
  • DN32 De40X4mm
  • DN40 De50X4mm

We typically use DN to label welded steel pipes, and rarely use De to mark pipes unless the wall thickness is involved. However, when it comes to labeling plastic pipes, it’s a different story altogether. It’s related to industry customs where, in practical construction process, we commonly refer to the 20, 25, 32, etc. pipes as De, not DN.

In addition, based on practical field experience:

a. The two types of pipe materials can be connected in two ways: threaded connections and flange connections.

b. Both galvanized steel pipes and PPR pipes can use the above two types of connections. However, pipes smaller than 50mm are more conveniently connected with threads, while those larger than 50mm are more reliably connected with flanges.

Note: When connecting two metal pipes of different materials, consider whether a galvanic reaction will occur. If it does, it will accelerate the corrosion rate of the active metal material pipe. It’s best to use flanged connections and insulating materials like rubber gaskets to separate the two metals. This includes using gaskets to separate bolts to avoid contact.

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The symbol Φ can also represent the outer diameter of a pipe, but in this case, it should be multiplied by the wall thickness.

3. Φ – Diameter of a Regular Circle

For example, Φ represents the diameter of a common circle. Of course, Φ can also represent the outer diameter of a pipe, but in this case, it should be multiplied by the wall thickness. For instance, Φ25×3 indicates a pipe with an outer diameter of 25mm and a wall thickness of 3mm.

For seamless steel pipes or non-ferrous metal pipes, they should be marked as “outer diameter × wall thickness”. For example, Φ107×4, where Φ can be omitted.

In China, ISO, and some Japanese standards, the wall thickness dimension is used to represent the pipe wall thickness series. For these types of pipes, the notation is outer diameter × wall thickness, such as Φ60.5×3.8.

Please refer to the pipe nominal diameter and outer diameter comparison table.

Comparison Table of Nominal Diameter and Outer Diameter for Various Types of Pipes
Serial Number  Nominal Diameter DN  Welded Steel Pipe Seamless Steel Pipe Spiral Pipe UPVC Pipe PP-R Pipe PB Pipe Aluminum Plastic Pipe Cast Iron Drainage Pipe High-Density Polyethylene Pipe 
11521.3   202020  
22026.828  252525  
32533.532  323232  
43242.338  404040  
5404848  505050  
6506057 506363635050
76575.576  757575 75
88088.589 7590909075 
9100114108 110110110110100110
10125140133     125 
11150165159 160   150160
12200 219219200   200200
13250 273273      
14300 325325      
15350 377377      
16400 426426      
17450 480480      
18500 530530      
19600 630630      

The Differences Between DN, De, Dg, and Φ

DN stands for Nominal Diameter, De for External Diameter, and Dg is a specially named Diameter Gong, which is exclusive to domestically made products in China and is no longer in use today.

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Relationships between DN, De, Dg, and Φ

  • De measures the external diameter of the pipe.
  • DN is the result of subtracting half of the pipe wall’s thickness from De.
  • Dg is generally not used.
  • Φ signifies the diameter of a regular circle.

The pipe diameter should be expressed in millimeters (mm), and the expression of the pipe diameter should conform to the following rules:

For gas transportation steel pipes (galvanized or non-galvanized), cast iron pipes, and other materials, the diameter should be represented by the Nominal Diameter (DN).

For seamless steel pipes, welded steel pipes (straight seam or spiral seam), copper pipes, stainless steel pipes, and other materials, the diameter should be represented as External Diameter × Wall Thickness (Φ).

For reinforced concrete (or concrete) pipes, clay pipes, acid-resistant ceramic pipes, cylinder tile pipes, and other materials, the diameter should be represented by the internal diameter (d).

For plastic pipes, the diameter should be represented according to the method specified in the product standards.

When the Nominal Diameter (DN) is used to represent the pipe diameter in the design, there should be a comparison table between the Nominal Diameter (DN) and the corresponding product specifications.

The specifications for PVC pipes used in building drainage are represented by de (Nominal External Diameter) × e (Nominal Wall Thickness) (GB 5836.1-92), and the specifications for PP (Polypropylene) pipes used for water supply are represented by de × e (Nominal External Diameter × Wall Thickness). This is how plastic pipes are marked on engineering drawings.

Metric Dimension Size (DN)

Generally referred to as the “nominal size,” DN doesn’t represent the external diameter nor the internal one. Instead, it represents the average of the two, often referred to as the average internal diameter.

For example, the metric dimension size (in mm) for a plastic pipe with an outer diameter of 63mm is DN50.

ISO Metric Dimension Size

The term ‘Da’ represents the outer diameter of PVC and ABS pipes, while ‘De’ represents the outer diameter of PP and PE pipes.

For instance, for a plastic pipe with an outer diameter of 63mm, the ISO metric dimension size (in mm) is Da63 for PVC and ABS pipes.

In summary, the respective ranges represented by De, DN, d, and φ are as follows:

  • De stands for the outer diameter of PPR, PE, and Polypropylene pipes.
  • DN refers to the nominal diameter of Polyvinyl Chloride (PVC) pipes, cast iron pipes, steel-plastic composite pipes, and galvanized steel pipes.
  • d represents the nominal diameter of concrete pipes.
  • φ should be marked as the “outer diameter × wall thickness” for seamless steel or non-ferrous metal pipes.

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