In simple terms, a rack unit is a standard measurement used to describe the vertical height of rack-mountable equipment. 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 U or RU) is a unit of measure defined as inches (44. A U is defined as 1 3/4 inches or 44. This article explains definition, planning, installation tips, and trends.