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

Water, gas transportation steel pipe (galvanized steel pipe or non-galvanized steel pipe), cast iron pipe, steel-plastic composite pipe and polyvinyl chloride (PVC) pipe and other pipes, should be marked with nominal diameter "DN" (such as DN15, DN50).

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

Third, D generally refers to the inner diameter of the pipe.

Four, 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, pipe diameter should be represented by the internal diameter d (such as d230, d380, etc.).

Five, Φ represents the diameter of a common circle; can also represent the outer diameter of the pipe, but this time should be multiplied by the wall thickness. Such as: Φ25 × 3, said the outer diameter of 25mm, wall thickness of 3mm pipe. For seamless steel pipes or non-ferrous metal pipes, the "outer diameter × wall thickness" should be marked. For example, Φ108×4, Φ may be omitted. Some steel pipe standards in China, ISO, and Japan use wall thickness to indicate the wall thickness of steel pipes. The specification for this type of steel pipe is the pipe outer diameter × wall thickness. For example, Φ60.5×3.8.