U

A rack unit, abbreviated "U," is a unit of measurement of the size of rack-mounted equipment. It measures how tall a rack-mountable piece of hardware is — 1U is equal to 1¾ inches, or 44.45 mm. A full-size rack cage is often around 42U tall, or just over six feet; half-height cages 18-22U tall are also common.

Most rack-mountable equipment is between 1 and 4U tall and 19 inches wide, although some telecommunications equipment uses special 23-inch-wide racks instead. Servers, disk storage arrays, and networking equipment like switches and firewalls are some of the most common rack-mounted devices, although all sorts of hardware can be found in rack-mountable versions. For example, four rack-mounted servers may all connect to a rack-mounted KVM switch to allow them to share a single keyboard, mouse, and monitor; they may also all plug into a single rack-mounted power strip and be kept cool by a rack-mounted cooling fan.

Using rack-mounted hardware saves space by stacking multiple pieces of equipment vertically and helps keep that equipment neat and organized. It simplifies cable management by keeping devices close to each other and locked in place. Since it simplifies organization, rack-mounted hardware is ubiquitous in data centers and other professional IT installations.