A rack unit (Unit, U) in a server rack is a standard unit of measurement used to define the height of equipment mounted in a rack. One rack unit equals 1.75 inches (44.45 mm).
The use of standardized rack units makes it possible to efficiently manage space and ensures convenient installation, replacement, and maintenance of equipment.
Racks and rack units of standard dimensions are widely used in data centers, server rooms, telecommunications facilities, and other environments where a large amount of equipment must be installed in a limited space. All rack-mount equipment intended for data centers—including servers, network switches, routers, and uninterruptible power supplies (UPS)—is designed and manufactured in accordance with this standard. This allows a high density of servers and network devices to be installed in one location while maintaining effective airflow and cooling.
Most modern data centers use racks with a standard height of 42U (the most common option) or 48U. This means, for example, that a 42U rack can accommodate equipment with a total height of up to 42 rack units.
42U 750 Rackmount Server Rack. Image: e-server.com.ua
Equipment in the SYSMATRIX GR 6642.900 42U server rack. Image: sysmatrix.ru