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A Rack Unit, often abbreviated as "U" (or sometimes just "RU"), is a standardized unit of measurement used to describe the height of equipment designed to be mounted in a standard 19-inch server rack or equipment rack. One Rack Unit is equivalent to 1.75 inches (44.45 mm) in height. Equipment that is mounted in racks, such as servers, networking gear, and storage devices, is typically specified in terms of Rack Units.