19-inch rack
Standardized frame for mounting electronic equipment modules.
Wikipedia / Wikimedia Commons
A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules, with each module having a front panel 19 inches (482.6 mm) wide. This dimension includes the ears that protrude from each side, allowing fastening to the rack frame. The format is widely used for computer servers, telecommunications equipment, networking hardware, audiovisual gear, professional audio equipment, and scientific equipment.
- standardized_width
- 19 inches (482.6 mm)
- rack_unit_height
- 1.75 inches (44.45 mm)
- typical_rack_height
- 42U
- first_standardized
- Around 1922 by AT&T
- common_uses
- Servers, telecom, networking, audio/video, scientific equipment
Lore & Background
The term 'relay rack' first appeared in the late 19th century in telephony, and by 1911 was used in railroad signaling, though early dimensions were not standardized. Around 1922, AT&T established the 19-inch rack format with rack units of 1.75 inches (44.45 mm) to reduce space for repeater and termination equipment in telephone central offices. Early repeaters from 1914 were installed ad hoc on shelves and in wooden boxes, but serial production led to custom racks. AT&T's engineering department redesigned the system into modular factory-assembled panels designed to mount on vertical supports spaced 19.5 inches between centers, with heights in multiples of 1.75 inches.
Reader's Guide
The 19-inch rack format has remained constant while the technology mounted within it has changed considerably, expanding into telecommunications, computing, audio, video, entertainment, and other industries. The EIA standard was revised in 1992 to comply with public law 100-418, setting the standard U as 44.45 mm. The rack allows dense hardware configurations without excessive floor space, and is used in enterprise data centers, ISP facilities, and professional server rooms. Mounting evolved from tapped-hole racks to clearance-hole racks with cage nuts, and later to square-hole racks allowing boltless mounting. Structural support includes two-post and four-post racks, with four-post racks common for servers, often using adjustable rails. Rack depth is not standardized, but common widths are 600 mm or 800 mm, and depths 600 mm, 800 mm, or 1,010 mm. Racks are often secured to floors or building structures, especially in seismic zones, as per Telcordia GR-63-CORE.
Did You Know?
- The 19-inch dimension includes the ears that protrude from each side of the equipment for fastening.
- The height of mountable equipment is standardized as multiples of 1.75 inches (44.45 mm), called one rack unit or U.
- The tapped-hole rack was first replaced by clearance-hole racks, which use cage nuts for easier maintenance.
- A 1U rackmount computer is not 1.750 inches tall but 1.719 inches, allowing a small gap for removal.
Frequently Asked Questions
Who standardized the 19-inch rack and when?
AT&T established the 19-inch rack standard around 1922, creating a common mounting frame so that different equipment modules could fit together in a single enclosure.
Exactly how wide is a 19-inch rack, and what does that measurement cover?
The front panel of each module is 482.6 mm (19 inches) wide, and that dimension already includes the side ears that extend outward to bolt the module onto the rack frame.
What is a rack unit (U) and how tall is a typical rack?
One rack unit equals 1.75 inches (44.45 mm) of vertical space, and a standard rack is usually 42U tall, giving roughly 73.5 inches of usable mounting height.
What kinds of equipment do people actually mount in a 19-inch rack?
You'll find computer servers, telecommunications gear, networking hardware, professional audio and video equipment, and scientific instruments all sharing the same 19-inch front-panel format.
Why is the 19-inch rack standard so important to modern infrastructure?
Because every manufacturer builds to the same 482.6 mm width and 1.75-inch unit spacing, operators can mix and match devices from different vendors in one frame, which makes data centers, broadcast studios, and labs far easier to build and scale.
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