Rack unit ethernet switchfrom wikiThe rack unit size is based on a standard rack specification as defined in EIA-310. The Eurocard specifies a standard rack unit as the unit of height; it also defines a similar unit, horizontal pitch (HP), used to measure the width of rack-mounted equipment. The standard was adopted worldwide as IEC 60297 Mechanical structures for electronic equipment – Dimensions of mechanical structures of the 482.6 mm (19 in) series, and defines the sizes for rack, subrack (a shelf-like chassis in which cards can be inserted), and the pitch of printed circuit boards/cards providing physical compatibility of technological equipment, typically in telecommunications.[citation needed]

While a rack unit is defined as 1+3⁄4 inches (44.45 mm), a front panel or filler panel in a rack is not an exact multiple of this height. To allow space between adjacent rack-mounted components, a panel is 1⁄32 inch (0.03125 in or 0.794 mm) less in height than the full number of rack units would imply. Thus, a 1U front panel would be 123⁄32 inch (1.71875 in or 43.66 mm) tall. If n is number of rack units, the ideal formula for panel height is h = (1.75n − 0.031) for calculating in inches, and h = (44.45n − 0.794) for calculating in millimetres. Manufacturing allows for dimensions with less precision.

The mounting-hole distance (as shown to the right) differs for 19-inch racks and 23-inch racks: The 19-inch racks uses uneven spacings (as shown to the right) while the 23-inch racks uses evenly spaced mounting holes. Although it is called a 19-inch rack unit, the actual mounting dimensions of a 19-inch rack unit are 185⁄16 inches (18.3125 in or 465.1 mm)[3] wide, center to center.

The 19-inch rack format with rack-units of 1.75 inches (44.45 mm) was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company central office.[4]