Rack screws are used to attach equipment to the rack angles of a 19″ rack. There are several sizes available, but 10-32, 12-24 and M6 are the most common. Exact threads can vary by manufacturer.
The number “10” is a size designator with no numerical meaning. The number “32” refers to 32 threads per inch. You can identify a 10-32 screw by measuring the diameter at exactly 3/16″ (4.76 mm).
12-24 rack screws are more common than 10-32 in pre-threaded racks. They are slightly larger and courser than 10-32 hardware. The number “12” is a size designator with no numerical meaning. The number “24” refers to 24 threads per inch. You can identify a 12-24 screw by measuring the diameter at slightly less than 7/32″ (5.55 mm).
M6 refers to a metric 6 mm screw. The outside diameter of the threads is 6 mm. The standard metric rack screw is actually an M6 x 0.1 mm. The number 0.1 mm refers to 0.1 threads per millimeter. You can identify an M6 screw by measuring the diameter at slightly more than 7/32″ (0.228″).