A Rack Unit (U or RU) is the standard height measurement used for mounting equipment in server racks. 5 inches tall, a 4U device is 7 inches tall, and so on. The total height of a rack is calculated by multiplying the number of U (rack units) by 1. This article explains definition, planning, installation tips, and trends. For example, a typical full-size rack cage is 42U high, while equipment is typically 1U, 2U, 3U, or 4U high. Important: U describes height only, but a server's real "capabilities" are also determined by chassis depth, internal layout, airflow, rails, power, and expansion (PCIe/risers, NVMe. The unit calculator below can convert rack U's into cm, inches and feet, which makes it a very useful tool for any installer or musician who needs to know exactly what equipment to buy when building a 19 inch rack. Scroll down for a complete table of rack U-to-inch/feet/cm values.
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