Rack unit (U)

A rack unit (U or RU) is the standard vertical measurement for equipment mounted in a data center rack, equal to 1.75 inches (44.45 mm). Server and network gear is sized in multiples: a 1U server is 1.75 inches tall, a 2U server 3.5 inches, and so on. The standard rack is 19 inches wide and most commonly 42U tall (about 1.87 m of usable space), though 45U, 48U, and 52U racks are increasingly common in high-density builds. Colocation providers sell space by the U for shared racks or by full cabinet. The EIA/ECIA-310 standard defines rack dimensions, ensuring interoperability across virtually all vendors' rack-mount hardware.