 The length of a machine screw or bolt is indicated by a following "x" and the length expressed in millimetres (e.g., M8-1.25x30 or M8x30).Tolerance classes defined in ISO 965-1 can be appended to these designations, if required (e.g., M500– 6g in external threads).External threads are designated by lowercase letter, g or h. Internal threads are designated by upper case letters, G or H.

ISO 261 specifies a detailed list of preferred combinations of outer diameter "D" and pitch "P" for ISO metric screw threads.