rack unit

A unit = 1¾ inches, indicating the amount of space taken up by a piece of electronic equipment in the mounting system described below. It is based on the height of the equipment's front panel; the width is standardized at 19 inches (482.6 mm). (Standard racks with widths of 23 inches (584 mm) and 30 inches (762 mm) are also made, but rarely if ever encountered by the consumer.)  The nominal height of the panel is a multiple of 1¾″ ; the size of the equipment is described by the number of 1¾″ units the equipment takes up.  Symbol, U, or sometimes RU.  A 3U panel would be (1¾″ × 3 = ) 5¼″ high.

This mounting system originated with the telephone company, who needed a standard for housing the millions of relays at one time used in the telephone system. Its adaptability, and the high quality and ready availability of components led to the system’s use in housing electronic equipment in industry and research, and in the 1980s by manufacturers of consumer audio equipment. The latter unfortunately sometimes made nonstandard sizes.

relay rack

tabletop rack

Courtesy Hammond Manufacturing.

The main parts of the rack itself are two uprights, usually steel. Down the center of each runs a series of holes tapped to accept 10-32 machine screws (sometimes 12-24). A clear space 17⅜ inches wide is left between the uprights. To mount a piece of equipment, its front panel is held against the uprights and screws passed through the panel into the holes in the uprights. Very heavy equipment may need additional support, such as angle brackets mounted to the rear of the uprights, or shelf supports running to additional uprights in the rear.

The holes in the upright occur in pairs with their centers ½ inch apart. The pairs are spaced with 1¼ inches from the center of the bottom hole in a pair to the center of the uppermost hole in the pair beneath. Some manufacturers add additional holes.

The 19″-wide front panels are generally aluminum 3⁄16″ thick or steel ⅛″ thick. The actual height of the panel is only a nominal multiple of 1¾″, because 1⁄64″ is taken off both top and bottom to provide a bit of clearance.

The sides of the panels are notched to accommodate the screws; these notches are ¼″ wide and end in a ¼″ hole whose center is 532" from the panel edge. The placement of the notches depends on the panel's height. The positions of the notches are always measured from the horizontal centerline of the panel.

1U and 2U panels have two notches on each side.  In a 1U panel the centers of the notches are ⅝″ above and below the centerline, and in a 2U panel, 1½″ above and below the centerline.

The 3U, 4U, and 5U panels also have a single pair of notches on each side, but they are much farther from the top and bottom edges, about 1½". The centers of the notches are the following distances above and below the centerline: 3U, 1⅛″; 4U, 2″; 5U, 2⅞″.

The standard 6U panel has 4 notches on each side. The centers of the notches nearest the centerline are 1½ inches above and below the centerline.  The centers of the other two notches are 2¼″ from the centers of the previously mentioned notches, placing their centers about 1½″ from the edge of the panel.

The 7U panel is 12¼″ high and has 6 notches on each end: the first 1⅛″ above and below the centerline, and then two more on 1¾″ centers. The centers of the highest and lowest notches are thus about 1½″ from the panel's edge.


The Hammond Manufacturing Company makes available a full set of engineering drawings of rack panels, in PDF or DXF format, dimensioned in decimal inches, at their website: www.hammondmfg.com


EIA-310D, Cabinets, Racks, Panels and Associated Equipment.  This revision Sept. 1992.

See also: DIN 41494 P1, Panel Mounting Racks for Electronic Equipments, and IEC 60297-1, Dimensions of Mechanical Structures of the 482.6 mm Series, Part 1: Panels and Racks.

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