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.

An HP ProLiant rack-mounted server that is 1U tall
An HP ProLiant rack-mounted server that is 1U tall

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.

Updated September 5, 2023 by Brian P.

quizTest Your Knowledge

Smartwatch to smartphone communication is an example of what type of network?

Correct! Incorrect!     View the WPAN definition.
More Quizzes →

The Tech Terms Computer Dictionary

The definition of U on this page is an original definition written by the TechTerms.com team. If you would like to reference this page or cite this definition, please use the green citation links above.

The goal of TechTerms.com is to explain computer terminology in a way that is easy to understand. We strive for simplicity and accuracy with every definition we publish. If you have feedback about this definition or would like to suggest a new technical term, please contact us.

Sign up for the free TechTerms Newsletter

How often would you like to receive an email?

You can unsubscribe or change your frequency setting at any time using the links available in each email.

Questions? Please contact us.