The “18U” designation refers to the unit height—each “U” equals 1. 45 mm), meaning the cabinet offers approximately 31. 5 inches (800 mm) of internal vertical space for mounting devices. Common server rack sizes are 19‑inch width, heights like 42U or 48U, and depths from ~24″ to 48″. Most IT environments default to 42U, 19-inch width, and 1000–1200 mm depth unless space constraints or special equipment dictate. Confused by 'U' (Rack Units) when looking at data cabinets? Use the following table to work out the height of a cabinet in inches, centimetres or feet based on the U height. Please note that the height of the cabinet/rack stated here is the useable internal 'working' height of the cabinet/rack. Other devices such as routers, UPS' and audio/video gear can also be housed.