A rack unit (abbreviated as U, less commonly seen as RU) is a unit of measurement applied to equipment racks and the servers, disk drives and other devices that they contain. One U is 1.75 inches (44.45mm); the standard rack, at 19 inches, is 42U.
Rack servers and other hardware designed to be rack-mounted are manufactured in multiples of 1.75 inches and specified in multiples of rack units, usually 1U, 2U, 3U or 4U. Racks are designed to hold equipment of those sizes. The holes in the mounting flanges of racks are arranged in groups of three, and that three-hole grouping is also called a rack unit.
Rack units for equipment is considered maximum dimensions. In practice, devices are often made slightly smaller than the specified U value to allow a little space. A device specified as 2U, for example, may in reality measure 3.44 inches in height, rather than the 3.5 inch multiple of 1.75.
This was last updated in October 2016
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A 19-inch rack is a standardized frame or enclosure for mounting multiple equipment modules. Each module has a front panel that is 19 inches (480 mm) wide, including edges or ears that protrude on each side which allow the module to be fastened to the rack frame with screws.
Overview and History
Equipment designed to be placed in a rack is typically described as rack-mount, rack-mount instrument, a rack mounted system, a rack mount chassis, sub-rack, rack mountable, or occasionally simply shelf. The height of the electronic modules is also standardized as multiples of 1.75 inches (44.45 mm) or one rack unit or “U”. Most racks are sold in the 42U form: that is, a single rack capable of holding forty-two 1U devices, or any combination of 1U, 2U, 3U or other rack unit heights that add up to 42 or less. Because of their origin as mounting systems for railroad signaling relays, they are still sometimes called relay racks, but the 19-inch rack format has remained a constant while the technology that is mounted within it has changed to completely different fields. The 19-inch (480 mm) standard rack arrangement is widely used throughout the telecommunication, computing, audio, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch (25 mm) centers, prevails in telecommunications.
19-inch racks are widely used for computer server equipment, allowing for dense hardware configurations without occupying excessive floor space or requiring shelving.
Typically, a piece of equipment being installed has a front panel height 1/32-inch (.031″) less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.75 inches (44 mm) tall but is 1.719 inches (43.7 mm) tall. 2U would be 3.469 inches (88.1 mm) instead of 3.5 inches (89 mm). This gap allows a bit of room above and below an installed piece of equipment, so it may be removed without binding on the adjacent equipment.
A Rack Unit (RU), also referred to as just U, is a measurement that refers to the height or space between shelves on a vertical storage rack designed to hold computer or AV equipment. Standard rack widths are 19 and 23 inches. The size of the rack mounted equipment is described by a number of units, such as 4U (i.e., 4 rack units of measure high).
This mounting system originated with the Telco industries need for a standard for housing the millions of relays at one time used for telephone system infrastructure. The system’s adaptability and the high quality and ready availability of components led to its use in housing electronic equipment in industry and research.
A front panel or filler panel in a rack is not an exact multiple of 1.75 inches (44.45 mm). To allow space between adjacent rackmounted components, a panel is 1⁄32 inch (0.031 inch or 0.79 mm) less in height than the full number of rack units would imply. Thus, a 1U front panel would be 1.719 inches (43.66 mm) high. If n is number of rack units, the formula for panel height is h = (1.750n − 0.031) inch = (44.45n − 0.79) mm.
The RU size is based on a standard rack specification as defined in the Electronic Industries Alliance specification EIA-310. The rack unit is also specified as the unit of height in the Eurocard standard, which also defines a similar unit, horizontal pitch (HP), used to measure the width of rack mounted equipment.
In the illustrated example, the whole distance (as pictured) is different for 19″ racks and 23″ racks: The 19″ racks uses odd spacing while the 23″ racks uses evenly spaced mounting holes.
CP Technologies specializes in rack mount computer systems for military and industrial applications.