DN (Nominal Diameter) means the nominal diameter of the pipe, which is the average of the outer diameter and the inner diameter. The value of  DN =the value of De -0.5*the value of  tube wall thickness. Note: This is neither the outer diameter nor the inner diameter.

Water, gas transmission steel pipe (galvanized steel pipe or non-galvanized steel pipe), cast iron pipe, steel-plastic composite pipe and polyvinyl chloride (PVC) pipe, etc., should be marked nominal diameter “DN” (such as DN15, DN50).

De (External Diameter) means the outer diameter of the pipe, PPR, PE pipe, polypropylene pipe outer diameter, generally marked with De, and all need to be marked as  the form as the outer diameter * wall thickness, for example De25 × 3.

D generally refers to the inner diameter of the pipe.

d generally refers to the inner diameter of the concrete pipe. Reinforced concrete (or concrete) pipes, clay pipes, acid-resistant ceramic pipes, cylinder tiles and other pipes, whose pipe diameter should be represented by the inner diameter d (such as d230, d380, etc.)

Φ represents the diameter of a common circle; it can also represent the outer diameter of the pipe, but this time it should be multiplied by the wall thickness.