We often talk about server racks in units of "U," but what exactly does "U" mean? Below, we've summarized some information about server rack dimensions. "U" is a unit representing the external dimensions of a server; it's an abbreviation for "unit." The detailed dimensions are determined by the Electronic Industries Association (EIA), an industry body.
The reason for specifying server dimensions is to ensure that servers are of appropriate size to fit in iron or aluminum racks. The racks have screw holes for mounting servers, allowing the server's screw holes to align, and then it is secured with screws, facilitating the installation of each server within the required space.
The specified dimensions are the server's width (48.26cm = 19 inches) and height (multiples of 4.445cm). Because the width is 19 inches, racks that meet this specification are sometimes referred to as "19-inch racks". The thickness is based on 4.445cm; 1U is 4.445cm.
