M6 refers to a metric 6mm screw. The outside diameter of the threads is 6mm. The standard metric rack screw is actually an M6 x 1mm. The number \u201c1mm\u201d refers to 1 threads per millimeter. ... You can identify an M6 screw by measuring the diameter with a ruler at slightly more than 7/32\u2033 (0.228\u2033).