Called with different names, a rack unit, U, or RU is a measurement unit, which defines the height of equipment that is designed to mount 1-inch rack, 23-inch rack. The 1U, 2U, 3U, 4U, 5U, 6U and 7U are all various sizes of rack mount servers, wherein 1 U is the smallest rack mount and 7U is the biggest. Both, the 19-inch and 23-inch dimension denotes the width of the rack mounting frame (including the width of the frame). Technically, the width of the equipment that needs to be mounted inside the rack is less. Usually, the width and height of the rackmount may be standard; however the depth of the rackmount may vary. 1 RU is equal to 1.75 inches (4.445 cm) high.
A 19-inch rack is considered as the standard frame for mounting multiple equipment modules, wherein each module has a 19 inches (482.6 mm) wide front panel. Broadly, a full size rack is 42U, and a typical ""half-height"" rack would be 18-22U. The RU or rack unit size is based on a standard rack specification as defined in EIA-310.