A rack unit, often abbreviated as U, is a standard measurement used to describe the height of equipment designed to be mounted in a 19-inch rack. One rack unit is equivalent to 1.75 inches (44.45 mm) in height. This measurement helps ensure that various devices, such as servers, switches, and routers, can fit properly within the rack. When equipment is specified in rack units, it allows for easy organization and management of space in data centers and server rooms. For example, a device that is 2U tall will occupy two rack units, taking up 3.5 inches of vertical space in the rack.