FREE USA Shipping $99+Our 4-hole rail design gives your gear 1/2RU vertical spacing for improved ventilation in your 19-inch rack.
EIA-310 Standard
The EIA-310 standard has served as the foundation for 19-inch equipment racks for over five decades. It defines the mechanical specifications for what is commonly referred to as a “standard rack,” including the Rack Unit height, vertical and horizontal hole spacing, rack opening dimensions, and front panel width. The standard also establishes precise tolerances for each of these critical measurements, ensuring interoperability and consistent fit across equipment and enclosures.
Equipment designed for installation in these racks is commonly referred to as rackmount or rack-mountable. The height of electronic modules is standardized in multiples of 1.75 inches or 44.45 millimeters, corresponding to one rack unit, commonly abbreviated as U.The 19-inch rack format was pioneered by AT&T around 1922 to optimize space for repeater and termination equipment in telephone central offices. Prior to standardization, early repeaters from 1914 were mounted in an ad-hoc manner on shelves, in wooden boxes, or custom-built racks. In response to rapid network growth, AT&T's engineering team developed a modular system with vertical supports spaced 19.5 inches apart and equipment panels with heights in precise multiples of 1.75 inches. This design laid the foundation for the standardized rack system still in use today.
By the mid-1930s, the standard had evolved to include tapped holes for 12-24 screws with alternating spacings of 1.25 inches and 0.5 inches. In 1992, the EIA standard was updated to align with federal requirements, precisely defining the rack unit height as 44.45 millimeters or 1.75 inches through a specific combination of 0.625 inch, 0.625 inch, and 0.5 inch segments.
The 19-inch rack format has demonstrated remarkable longevity, remaining essentially unchanged while the technology it houses has evolved dramatically. Its applications have expanded well beyond its original telephone industry roots, now serving as the standard for telecommunications, computing, broadcasting, audio-video systems, and many other sectors. Although the 23-inch Western Electric standard persists in certain legacy telephone facilities, the 19-inch format continues to dominate across most industries worldwide.
Nineteen-inch racks, available in both two-post and four-post configurations, form the backbone of modern data centers, ISP facilities, and corporate server rooms. Their standardized design enables dense, organized hardware installations while minimizing floor space requirements.
Beyond IT infrastructure, nineteen-inch racks are widely used to house professional audio and video equipment, such as amplifiers, signal processors, interfaces, and mixing consoles. They are also commonly employed for industrial power, control, and automation systems.
Equipment front panels are intentionally manufactured 1/32 inch, or 0.794 millimeters, shorter than the nominal rack unit height. A 1U device, for example, measures approximately 1.719 inches or 43.66 millimeters in height rather than the full 1.75 inches.
The standard formula for calculating panel height is 1.75n minus 0.031 inches, or 44.45n minus 0.794 millimeters, where n represents the number of rack units. This clearance prevents binding between adjacent units, allowing for easier installation and removal.
Known Issues with Hole Spacing
Standard 19-inch racks use a specific hole pattern on the mounting rails. Holes are grouped in sets of three, with each group representing one Rack Unit, commonly called 1U. Each U measures 1.75 inches (44.45 mm) vertically.
The spacing is measured center-hole to center-hole and remains consistent whether the rack has square or round mounting holes.
Because of this uneven spacing pattern, it's easy to accidentally install slide rail assemblies "off a U" — meaning the hooks or holes on the rail don't line up with the holes in the rack. This is one of the most common installation issues.
