Dell 8 Installation Manual page 14

Figure 1-3. One Rack Unit
1 U (44 mm or 1.75 inches)
2 Mark the rack's front vertical rails with a felt-tipped pen or masking tape
approximately 44.45 cm (17.5 inches) above the original mark you made
(or count up 30 holes in a rack that meets CEA-310-E standards).
(If you counted holes, place a mark just above the top hole.) This mark
or piece of tape indicates where the system's upper edge will be located
on the vertical rails (see Figure 1-4).
12
Rack Installation Guide
12.7 mm (0.5 inch)
15.9 mm (0.625 inch)
15.9 mm (0.625 inch)
12.7 mm (0.5 inch)