This article focuses mainly on “tube diameter”, a rather perplexing subject due to its various methods of representation. I will explain the use and differences between the outer diameter (De), the diameter/outer diameter of a regular circle (Φ) and the nominal diameter (DN).
What do DN, De and Φ represent?
1 . DN – Nominal Tube Diameter
Commonly used to describe: galvanized steel pipes
The nominal diameter can be represented in metric mm or inches. Pipe fittings also use nominal diameter for representation, with the same implications as seam pipes. Its correlation with the imperial system is as follows:
| Inches (in.) | DN | Nominal External Diameter (mm) |
| 1/2（4″） | 15 | Φ20 |
| 3/4（6″） | 20 | Φ25 |
| 1 | 25 | Φ32 |
| 4/11 | 32 | Φ40 |
| 2/11 | 40 | Φ50 |
| two | 50 | Φ63 |
| 2/21 | 65 | Φ75 |
| 3 | 80 | Φ90 |
| 4 | 100 | Φ110 |
| 5 | 125 | Φ140 |
| 6 | 150 | Φ160 |
The nominal diameter, also known as average external diameter, originates from the fact that metallic tubes have thin walls and the difference between the external diameter and the internal diameter is insignificant. Therefore, the average of the outer diameter and inner diameter of the pipe is used as the nominal diameter.
DN means nominal diameter, which is the universal gauge of various pipes and pipe fittings. The nominal diameter is a round integer convenient for reference, which is only loosely related to the processing size. Pipes and pipe fittings with the same nominal diameter can be exchanged. 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 the outer diameter nor the inner diameter, but a nominal size close to the inner diameter of common steel pipes. Each nominal diameter corresponds to an external diameter, and its internal diameter varies with different thicknesses.
2. From – External Diameter of the Tube
De mainly 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 indicated. Pipes usually marked De need to be labeled in the format OD X wall thickness.
Take 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 wall thickness is involved. However, when it comes to labeling plastic pipes, it's a completely different story. It is related to industry customs where, in the practical construction process, we commonly refer to tubes 20, 25, 32, etc. like De, not DN.
Additionally, based on practical field experience:
The. The two types of pipe materials can be connected in two ways: threaded connections and flanged connections.
B. Both galvanized steel pipes and PPR pipes can use the above two types of connections. However, pipes smaller than 50 mm are more conveniently connected with threads, while those larger than 50 mm are more reliably connected with flanges.
Note: When connecting two metal pipes of different materials, consider whether a galvanic reaction will occur. If this happens, it will accelerate the corrosion rate of the active metal material pipe. It is best to use flanged connections and insulating materials such as rubber gaskets to separate the two metals. This includes using gaskets to separate screws to prevent contact.
The symbol Φ can also represent the external diameter of a pipe, but in this case it must be multiplied by the wall thickness.
3. Φ – Diameter of a Regular Circle
For example, Φ represents the diameter of an ordinary circle. Of course, Φ can also represent the outer diameter of a pipe, but in this case it must be multiplied by the wall thickness. For example, Φ25×3 indicates a pipe with an outer diameter of 25 mm and a wall thickness of 3 mm.
For seamless steel pipes or non-ferrous metal pipes, they should be marked as “outer diameter x 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 outside diameter × wall thickness, such as Φ60.5×3.8.
Refer to the nominal pipe 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 Tube | Spiral Tube | UPVC pipe | PP-R Pipe | PB tube | Aluminum Plastic Tube | Cast Iron Drain Pipe | High Density Polyethylene Pipe |
| 1 | 15 | 21.3 | 20 | 20 | 20 | |||||
| two | 20 | 26.8 | 28 | 25 | 25 | 25 | ||||
| 3 | 25 | 33.5 | 32 | 32 | 32 | 32 | ||||
| 4 | 32 | 42.3 | 38 | 40 | 40 | 40 | ||||
| 5 | 40 | 48 | 48 | 50 | 50 | 50 | ||||
| 6 | 50 | 60 | 57 | 50 | 63 | 63 | 63 | 50 | 50 | |
| 7 | 65 | 75.5 | 76 | 75 | 75 | 75 | 75 | |||
| 8 | 80 | 88.5 | 89 | 75 | 90 | 90 | 90 | 75 | ||
| 9 | 100 | 114 | 108 | 110 | 110 | 110 | 110 | 100 | 110 | |
| 10 | 125 | 140 | 133 | 125 | ||||||
| 11 | 150 | 165 | 159 | 160 | 150 | 160 | ||||
| 12 | 200 | 219 | 219 | 200 | 200 | 200 | ||||
| 13 | 250 | 273 | 273 | |||||||
| 14 | 300 | 325 | 325 | |||||||
| 15 | 350 | 377 | 377 | |||||||
| 16 | 400 | 426 | 426 | |||||||
| 17 | 450 | 480 | 480 | |||||||
| 18 | 500 | 530 | 530 | |||||||
| 19 | 600 | 630 | 630 | |||||||
The differences between DN, De, Dg and Φ
DN stands for Nominal Diameter, De stands for Outer Diameter, and Dg is a specially named Diameter Gong exclusive to products manufactured domestically in China and is no longer used today.
Relationships between DN, De, Dg and Φ
- Measure the outer diameter of the tube.
- DN is the result of subtracting half the tube wall thickness from De.
- Dg is generally not used.
- Φ means the diameter of a regular circle.
The tube diameter must be expressed in millimeters (mm), and the expression of the tube diameter must comply with the following rules:
For steel pipes for gas transport (galvanized or non-galvanized), cast iron pipes and other materials, the diameter must be represented by the Nominal Diameter (DN).
For seamless steel pipes, welded steel pipes (straight or spiral seam), copper pipes, stainless steel pipes and other materials, the diameter should be represented as Outer Diameter × Wall Thickness (Φ).
For reinforced concrete (or concrete) pipes, clay pipes, acid-resistant ceramic pipes, cylindrical pipes and other materials, the diameter should be represented by the inner diameter (d).
For plastic pipes, the diameter must 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 project, there must be a comparison table between the Nominal Diameter (DN) and the corresponding product specifications.
The specifications for PVC pipes used for 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)
Often referred to as “nominal size”, DN represents neither the outer nor the inner diameter. Instead, it represents the average of the two, often referred to as the average inside diameter.
For example, the metric dimension size (in mm) for a plastic pipe with an outer diameter of 63 mm 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 example, for a plastic pipe with an outer diameter of 63 mm, the ISO metric dimension size (in mm) is Da63 for PVC and ABS pipes.
In summary, the respective intervals represented by De, DN, de φ are the following:
- De means 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 the concrete pipes.
- φ shall be marked as “outer diameter x wall thickness” for seamless steel or non-ferrous metal pipes.
