In the context of data centers and server racks, ‘U’ is shorthand for a single ‘rack unit’. It is a unit of measure used to describe the height of equipment intended for mounting in a 19-inch or 23-inch rack. One rack unit (1U) is defined as 1.75 inches (44.45mm) in height.

Rack units are a standardized measurement used to define the vertical space available in a rack and the size of the equipment (such as servers, switches, and other rack-mountable devices) that can be placed within that space. The notation 1U, 2U, 3U and so on is used to specify the height of rack-mounted equipment.

For example:

• A 1U server would be 1.75 inches tall.

• A 2U server would be 3.5 inches tall (2 x 1.75 inches).

• A 4U server would be 7 inches tall (4 x 1.75 inches), and so on.

The height of a server is often an indication of its capabilities, as more space typically allows for more hardware components. However, advancements in technology have made it possible for powerful servers to come in smaller form factors. Racks themselves come in various sizes, with a standard full-size rack typically being 42U tall, although there are racks as small as single-digit Us and as large as 48U or more.