A Rack Unit (RU), also referred to as just U, is a measurement that refers to the height or space between shelves on a vertical storage rack designed to hold computer or AV equipment. Standard rack widths are 19 and 23 inches. The size of the rack mounted equipment is described by a number of units, such as 4U (i.e., 4 rack units of measure high).


Rack Mount 'U' Defined1U of rack space is equal to 1.75 inches. For example, a rack designated as 20U has 20 rack spaces for equipment and has 35 inches (20 x 1.75”) of vertical usable space. Rack, cabinet spaces, and the equipment which fit into them are all measured in U.


This mounting system originated with the Telco industries need for a standard for housing the millions of relays at one time used for telephone system infrastructure. The system’s adaptability and the high quality and ready availability of components led to its use in housing electronic equipment in industry and research.


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 rackmounted components, a panel is 132 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.


The RU size is based on a standard rack specification as defined in the Electronic Industries Alliance specification 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.


In the illustrated example, the whole distance (as pictured) is different for 19″ racks and 23″ racks: The 19″ racks uses odd spacing while the 23″ racks uses evenly spaced mounting holes.


Chassis Plans specializes in rack mount computer systems for military and industrial applications.