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A rack unit, U or RU is a unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack or a 23-inch rack (The 19-inch or 23-inch dimension refers to the width of the equipment mounting frame in the rack i.e. the width of the equipment that can be mounted inside the rack). One rack unit is 1.75 inches (44.45 mm) high.
The size of a piece of rack-mounted equipment is frequently described as a number in "U". For example, one rack unit is often referred to as "1U", 2 rack units as "2U" and so on.
A typical full size rack is 44U, which means it holds just over 6 feet of equipment, and a typical "half-height" rack would be 18-22U, or around 3 feet high.
The term "half-rack" is used differently in computing and audio/video (A/V) applications:
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, an obsolete Russian length unit.
The rack unit size is based on a standard rack specification as defined in 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.
Do note that the mounting hole distance (as shown to the right) is different for 19" racks and 23" racks: The 19" racks uses odd spacings (as shown to the right) while the 23" racks uses evenly spaced mounting holes.
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