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. 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 height of a rack is measured in Rack Units, written as U. A U is defined as 1 3/4 inches or 44. This article explains definition, planning, installation tips, and trends. This standardization allows IT equipment like servers, switches, routers, and patch. Q: What is 1U in a server rack? A: 1U refers to one rack unit, which equals 1. Q: How tall is a 2U server? A: A 2U server is 3.