ISO 4065:2018 specifies the relationship between the nominal wall thickness en and the nominal outside diameter dn of thermoplastics pipes. ISO 4065:2018 is applicable to solid-wall thermoplastics pipes of constant circular cross-section along the whole length of the pipe, whatever their method of manufacture, their composition or their intended application. NOTE Pipes with constant circular cross-section along the whole length of the pipe are commonly known as pipes with smooth external and internal surfaces.