Rack unit
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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 server 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.
The rack unit size is based on a standard rack specification as defined in EIA-310.
