Rack mounted hardware is installed in two standard sized racks or cabinets. Those sizes are:

The 19-inch (482mm) and 23-inch (584mm) rack dimensions are the two standard sizes defined by EIA-310 (Cabinets, Racks, Panels, and Associated Equipment), which is the foundational standard for equipment rack dimensions in telecommunications and data center environments. The 19-inch rack is the dominant standard in modern IT and network environments, while the 23-inch rack is used primarily in legacy telecommunications carrier applications.

762mm (30 inches) is not a standard rack width; only 19-inch and 23-inch widths are recognized in EIA-310 and related telecommunications standards.

Neither 610mm (24 inches) nor 762mm (30 inches) correspond to any recognized EIA, ANSI, or IEC standard rack mounting width for telecommunications or IT equipment.

762mm (30 inches) and 914mm (36 inches) are not defined as standard rack widths in any recognized industry standard for equipment cabinets or racks.

1020mm (40 inches) is not a standard rack dimension; the only two standardized rack widths are 19 inches and 23 inches as defined by EIA-310.