The 1U, 2U, 3U, 4U, 5U, 6U and 7U are all different sized rackmount servers, and the "U" following the number is short for unit. The number indicates the size of the rackmount, 1U being the smallest rackmount and 7U being the biggest rackmount. A “Rack Unit” (U) is a standard height measure for mounting equipment in a server rack. The term is used throughout the IT, telecom, AV, and data center industries. One rack unit (1U) equals exactly 1. The picture, courtesy of Dell Inc. Important: U describes height only, but a server's real "capabilities" are also determined by chassis depth, internal layout, airflow, rails, power, and expansion (PCIe/risers, NVMe. A rack unit (abbreviated U or RU) is a unit of measure defined as 13⁄4 inches (44. It is most frequently used as a measurement of the overall height of 19-inch and 23-inch rack frames, as well as the height of equipment that mounts in these frames, whereby the height of the frame or. A “U” rack dimension stands for a rack unit and refers to the height of a device that can be mounted in the server rack. For larger hardware, sizes greater than 1U are used.
