These standards are essentially the same, but developed at different times at different locations. They are essential for racks because they control the critical dimensions. These dimensions must be the same around the world, so that equipment manufactured by any number of companies can be installed into any rack. The critical dimensions are:

- Width from hole of mounting channel to mounting channel (panel mounting) is 465mm.

- Width between the mounting rails (panel aperture) is 450mm.

- Pitch between vertical holes (Universal Spacing) set by the standards and measured in “U” standards. One U is equal to 44.45 mm. These holes are either square or round. In the USA , round holes are very common, but these are mainly used in telecommunications applications. These round holes must be either tapped or use special clip nuts to mount the equipment.
The overall outside width and depth are not specified, but common widths are 600mm and 800mm while common depths are 600, 800 and 900+.

         The height of a rack is measured in number of U's. We will refer to “42U” or “27U” etc. This is the inside useable height of the rack. Remember one U is equal to 44.45mm, so a 42U high rack is over 1.8 meters.
(42 x 44.45 = 1,867 plus additional height for the base and top).

Note: The Japanese (JIS) and German (DIN) standards use a different spacing. The pitch is a standard 50 mm between centers of all holes.
In addition to the 19” racks, there are also 23” types. These are used for specialized equipment installations and are not covered in this paper.