Rack unit

A typical section of rack rail, showing rack unit distribution.

A rack unit or U (less commonly 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.

The term "half-rack" is used differently in computing and audio/video (AV) applications.

• In AV, half-rack units typically describe units that fit 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.

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.

External links

• Abstract of EIA-310 standard at Electronic Industries Alliance website