19-inch rack

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The 19-inch rack format, originally used as mounting systems for railroad signaling relays,[1] has remained a constant while the technology that is mounted within it has changed to completely different fields. Equipment designed to be placed in a rack is typically described as rack-mount, rack mountable or subrack. It is covered by standards DIN 41494, IEC 297-3 and IEEE 1001.1


19" rack dimensions
Rack rail and cage nut

RU (rack unit) or U is the unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack. One U is 1.75 inches (44.45 mm) high.

HP (horizontal pitch) is the unit of length defined by the Eurocard standard used to measure the horizontal width of rack mounted electronic equipment. One HP is 0.2 inches (5.08 mm) wide. A standard 19-inch rack is 84 HP wide.

A rack's mounting fixture consists of two parallel vertical metal "posts" or "panel mounts". The posts are each 0.625 inches (15.88 mm) wide, and separated by a gap of 17.75 inches (450.85 mm), giving an overall rack width of 19 inches (482.60 mm). The posts have matching holes in them at regular intervals, with a center-to-center distance of 18.312 inches (465.12 mm). Vertically the holes in the posts are arranged in repeating sets of three, with center-to-center separations of 0.5 inches (12.70 mm), 0.625 inches (15.88 mm), 0.625 inches (15.88 mm). The hole pattern thus repeats every 1.75 inches (44.45 mm).


Tapped-hole racks are used for hardware that rarely changes, such as phone, network cabling panels, TV broadcasting facilities, studios and relay racks. Clearance-hole racks have holes large enough to permit a bolt to be freely inserted through without binding, and held in place by cage nuts, (a spring steel clip within which is a captive nut). Square-hole racks allow boltless mounting, such that the rack-mount equipment only needs to insert through and hook down into the lip of the square hole.

Mounting screws

These days square holes for cage-nuts in rack rails are more common than threaded holes. The mounting screws and cage-nuts sizes are typically:

  • Metric M6, (thread size of 6 millimeters). These are larger and stronger than the #10-32 screws.
  • American #10-32, (drill size 10 and 32 threads per inch), smaller than a #12-24 screw.
  • American #12-24, (drill size 12 and 24 threads per inch), larger than a #10-32 screw, smaller than an M6 screw.[2]


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