5 inches tall, a 4U device is 7 inches tall, and so on. The “U” standard makes it easy to calculate how many pieces of equipment will fit in a rack and helps maintain consistency across different brands and. One rack unit equals 1. This article explains definition, planning, installation tips, and trends. 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. 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. We explain what 1U, 2U, 18U, 42U, and other configurations mean, discussing precis Decoding Rack Units: Your Essential Guide to.