From left to right: a #6-32 UNC thumbscrew, a #6-32 UNC screw, an M3 screw and a self-tapping screw for case fans
Computer case screws are the hardware used to secure parts of a PC to the case. Although there are numerous manufacturers of computer cases, they have generally used three thread sizes. The Unified Thread Standard (UTS) originates from the United States, while the ISO metric screw thread is standardized worldwide. In turn, these thread standards define preferred size combinations that are based on generic units—some on the inch and others on the millimetre.

The #6-32 UNC screws are often found on 3.5″ hard disk drives and the case’s body to secure the covers. The M3 threaded holes are often found on 5.25″ optical disc drives, 3.5″ floppy drives, and 2.5″ drives. Motherboards and other circuit boards often use a #6-32 UNC standoff. #4-40 UNC thumb screws are often found on the ends of DVI, VGA, serial and parallel connectors.

More modern cases from certain manufacturers (Dell, Gateway) and enthusiast cases will lack screws altogether, instead utilizing a tool-less design