Standard 19-inch racks use a vertical hole spacing of 1.75 inches per rack unit (U) with a three-hole set pattern, and horizontal rail spacing of 17.75 inches between mounting rails.

Vertical Hole Spacing

The vertical mounting holes on a 19-inch rack are arranged according to the EIA-310 standard. Each rack unit (1U) measures 1.75 inches (44.45 mm) in height. Within each 1U, the holes are grouped in a three-hole set with the following center-to-center spacing:

• Bottom hole to middle hole: 0.625 inches (15.9 mm)
• Middle hole to top hole: 0.625 inches (15.9 mm)
• Top hole to next U: 0.5 inches (12.67 mm) This pattern repeats for each rack unit, ensuring consistent alignment for rack-mounted equipment ( ).

Horizontal Hole Spacing

The horizontal distance between the vertical columns of mounting holes (the front rails) is 17.75 inches (451 mm) internally. The overall front panel width of the rack is 19 inches (482.6 mm), allowing for handles or bezels on equipment ( ). The holes themselves typically have a diameter of 0.28 inches (7.1 mm) to accommodate standard screws ( ).

Additional Notes

• The three-hole set design allows for half-unit (0.5U) spacing in some racks, improving ventilation and flexibility ( ).
• Both square and round holes follow the same center-to-center spacing, so equipment designed for either type will fit properly.
• For four-post racks, the distance between front and rear rails varies depending on depth, typically 23 to 30 inches, to accommodate rear mounting brackets ( ). Understanding these measurements ensures proper installation of servers, switches, and other rack-mounted devices while maintaining compliance with industry standards.