ISO 161-1:2018 specifies the nominal outside diameters for metric thermoplastics pipes for the conveyance of fluids in pressure and non-pressure applications. It also specifies nominal pressure ratings and minimum required strengths for pressure applications. ISO 161-1:2018 is applicable to smooth thermoplastics pipes of constant circular cross-section along the whole length of the pipe, whatever their method of manufacture or material of construction. ISO 161-1:2018 is not applicable to pipes designated according to the nominal inside diameter DN/ID. ISO 161-1:2018 is a basis for standards writers for the selection of nominal diameters and nominal pressures in the drafting of product standards.