Rack Unit

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 RU 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:

  • In A/V, "half-rack" typically describes equipment that fits in a certain number of rack units, but occupy only half the width of a 19-inch rack (9.5 in or 241 mm). These are commonly used when a piece of equipment does not require full rack width but may require more than 1U of height. For example, a "4U half-rack" DVCAM deck would occupy 4U (7 in) height × 9.5 in width, and in theory, two 4U half-rack decks could be mounted side by side and occupy the 4U space.
  • In computing and information technology, however, "half-rack" typically describes a unit that is 1U high and half the depth of a 4-post rack (such as a network switch, router, KVM switch, or server), such that two units can be mounted in 1U of space (one mounted at the front of the rack and one at the rear). When used to describe the rack enclosure itself, the term "half-rack" typically means a rack enclosure that is 24U tall.

Rack Unit Front Panel

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 and 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.

RU Mounting Hole Distance

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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