Meanings and Differences of Pipe Diameter Symbols: DN, De, D, d, Φ

DN refers to the nominal diameter of a pipe.

Note: This is neither the outer diameter nor the inner diameter. It is related to the early development of pipe engineering and imperial units. It is commonly used to describe galvanized steel pipes. The relationship with imperial units is as follows:

• 1/2 inch pipe (1/2 inch): DN15

• 3/4 inch pipe (3/4 inch): DN20

• 1 inch pipe: DN25

• 1¼ inch pipe: DN32

• 1½ inch pipe: DN40

• 2 inch pipe: DN50

• 3 inch pipe: DN80 (also marked as DN75 in many places)

• 4 inch pipe: DN100

De mainly refers to the outer diameter of a pipe.

When De is used, it should be marked in the form of outer diameter × wall thickness. It is mainly used for seamless steel pipes, PVC and other plastic pipes, and other pipe materials where wall thickness needs to be specified.

Take galvanized welded steel pipes as an example. The two marking methods (DN and De) are as follows:

• DN20 → De25×2.5mm

• DN25 → De32×3mm

• DN32 → De40×4mm

• DN40 → De50×4mm

• etc.

We are accustomed to using DN to mark welded steel pipes. When wall thickness is not involved, we rarely use De to mark pipes. However, marking plastic pipes is a different story – it again depends on industry habits. In actual construction, when we casually refer to pipes as 20, 25, 32, etc., we mean De, not DN. There is a size difference here. Failing to understand this can easily lead to losses during procurement and construction.

The connection methods for these two types of pipe materials are basically thread connection and flange connection. Other connection methods are rarely used.

Galvanized steel pipes and PPR pipes can both use the above two connection methods. For pipes smaller than DN50, thread connection is more convenient; for pipes larger than DN50, flange connection is more reliable.

Note: If two metal pipes of different materials are connected, consider whether a galvanic reaction may occur, which would accelerate the corrosion of the more active metal pipe. It is best to use a flange connection and separate the two metals with an insulating material such as a rubber gasket. Even the bolts should be isolated with gaskets to avoid contact.

Scope of Use for De, DN, d, and φ

• De – Outer diameter of PPR, PE, polypropylene pipes

• DN – Nominal diameter of polyethylene (PVC) pipes, cast iron pipes, steel-plastic composite pipes, galvanized steel pipes

• d – Nominal diameter of concrete pipes

• φ – Nominal diameter of seamless steel pipes. The specification is given as, for example, φ100: 108×4

Common Conversions (DN to approximate outer diameter in mm)

DN

Approx. φ (mm)

Alternative conversion table (slightly different for some pipe types):

DN

Approx. φ (mm)