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Reference Number: AA-00970// Views: AA-00970// Created: 2015-06-30 19:24:47// Last Updated: 2015-06-30 19:31:40
General Article
Rack Units 1U 2U 4U


What is a rack unit? What is the measurement 1U 2U 4U refer to in regards to a 19 inch OPAL-RT HIL system?

Source: Wikipedia.


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.

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.

Rack Unit

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

Also referred to as 1RU, although RU is not technically correct.

U is the standard unit of measure for designating the vertical usable space, or height of racks (metal frame designed to hold hardware devices) and cabinets (enclosures with one or more doors). This unit of measurement refers to the space between shelves on a rack. 1U is equal to 1.75 inches. For example, a rack designated as 20U, has 20 rack spaces for equipment and has 35 (20 x 1.75.) inches of vertical usable space. Rack and cabinet spaces and the equipment which fit into them are all measured in U.

  • Rackmount:

  • rack unit:

  • Rack: