About Servers
A 19-inch rack is a standardized (EIA 310-D, IEC 60297 and DIN 41494 SC48D) system for mounting various electronic modules in a "stack", or rack, 19 inches (482.6 mm) wide. Equipment designed to be placed in a rack is typically described as rack-mount, a rack mounted system, a rack mount chassis, subrack, or occasionally, simply shelf. The slang expression for a subrack (generally 1U = 1.75 in = 44.45 mm height) is "pizza box" due to the similarity in size and shape, see also pizza box form factor. Most racks are sold in the 42U form: that is, a single rack capable of holding 42 1U pizza box servers.
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. This 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 centers, prevails in telecommunications.
Typically, a piece of equipment being installed has a front panel height 1/32-inch (.031") less than the allotted number of U's. Thus, a 1U rackmount computer is not 1.75-inches tall but is 1.729-inches tall. 2U would be 3.469-inches instead of 3.5-inches. 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 is a unit of measure used to describe the height of a server, network switch or other similar device mounted in a 19-inch rack or a 23-inch rack (refers to width of rack). One rack unit is 44.45 mm (1.75 in) high.
One rack unit is commonly designated as "1U"; similarly, 2 rack units are "2U" and so on. The size of a piece of rack mounted equipment is usually described as a number in "U". One rack unit is also sometimes referred to as "1RU"; most racks have 42 U of height.
Professional audio and video gear often comes with rack mount options and use the same measurement specifications.
Half-rack units typically describe units that fit in a certain number of rack units, but occupy only half the width of the rack (9.5 inches or 241 mm). For example, a "4U half-rack" DVCAM deck would occupy 4U (4 × 1.75 inches) height × 9.5 inches width. In theory, two half-rack decks could occupy the 4U space.
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 rack mounted 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.
Coincidentally, a rack unit is equal to a vershok, which is an obsolete Russian length unit.