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The EIA-310 specification has been the 19-inch racks standard for equipment for 50 years. EIA-310 is a specification for what is often called the “standard rack”. This specification standardizes several important features of 19-inch racks, such as the Rack Unit (RU or U), vertical hole spacing, horizontal hole spacing, rack opening and front panel width. The specification also sets tolerances on each of these dimensions.
Equipment designed to be placed in a rack is typically described as rackmount, 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 (less commonly RU).
The 19-inch rack format with rack-units of 1.75 inches (44.45 mm) was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company central office. The earliest repeaters from 1914 were installed in ad-hoc fashion on shelves, in wooden boxes and cabinets. Once serial production started, they were built into custom-made racks, one per repeater. But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 19?1/2 inches between centers. The height of the different panels will vary, ... but ... in all cases to be a whole multiple of 1-3/4 inches".
By 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of 1.25 inches (31.75 mm) and 0.5 inches (12.70 mm)  The EIA standard was revised again in 1992 to comply with the 1988 public law 100-418, setting the standard U as 15.875 mm (0.625 in) + 15.875 mm (0.625 in) + 12.7 mm (0.500 in), making each "U" 44.45 millimeters (1.75 in).
The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably and the set of fields to which racks are applied has greatly expanded. The 19-inch (482.6 mm) standard rack arrangement is widely used throughout the telecommunication, computing, audio, video, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch (25.4 mm) centers, is still used in legacy ILEC/CLEC facilities.
Nineteen-inch racks in two-post or four-post form hold most equipment in modern data centers, ISP facilities, and professionally designed corporate server rooms. They allow for dense hardware configurations without occupying excessive floor space or requiring shelving.
Nineteen-inch racks are also often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and even small-scale audio mixers. A third common use for rack-mounted equipment is industrial power, control, and automation hardware.
Typically, a piece of equipment being installed has a front panel height 1/32 inch (0.03125 inches or 0.794 millimeters) less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.75 inches (44.45 mm) tall but is 1.71875 inches (43.66 mm) tall. If n is number of rack units, the ideal formula for panel height is h = (1.75n - 0.031) for calculating in inches, and h = (44.45n - 0.794) for calculating in millimeters. 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.
Wikipedia contributors. "19-inch rack." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 8 Dec. 2019. Web. 9 Dec. 2019.
Known issues with hole spacing
The hole spacing for standard 19-inch racks on the mounting flange is spaced in groups of three holes. This three-hole group is defined as a Rack Unit (RU) or sometimes just called a “U”. 1U occupies 1.75 inches (44.45 mm) of vertical space. Manufacturers of rackmount equipment make their product lines based upon how many RUs that the equipment occupies in the server rack. The three-hole group spacing is measured from center hole to center hole and does not vary, regardless of whether the server rack has square or round mounting holes. This uneven hole spacing can cause installation issues if the installer does not pay close attention to where the slide rail assemblies are being installed. The most common problem is that the slide rail assembly hooks or holes do not line up with the holes in the server rack. This is typically due to the slide rail assemblies being misaligned with the hole pattern or otherwise referred to as “being off a U”.
“Define: Rack Unit ‘U’ or ‘RU.’” The Server Rack FAQ RSS, https://www.server-racks.com/rack-unit-u-ru.html.