Each "U" represents a standard space of 19 inches (48. This size is widely used in servers and network devices. For example, a typical full-size rack cage is 42U high, while equipment is typically 1U, 2U, 3U, or 4U high. The Eurocard specifies a standard rack unit as the unit of height; it also defines a similar unit. A rack unit, abbreviated as U (or RU), is a standardized unit of measurement used to describe the vertical space occupied by equipment in a server rack. Height (in inches) = Rack Units (U) × 1.