Most racks are designed to hold equipment that’s 19" (48.3 cm) wide. But
height and depth may vary from rack to rack. Common rack heights range
from 39" (99.1 cm) to 87" (221 cm). One rack unit (1U) is 1.75 inches of usable vertical space. So, for example,
a rackmount device that’s 2U high will take up 3.5 inches of rack space.
A rack that’s 20U high will have 35 inches of usable space.
1U = 1.75" ( 1 Rack Unit (1U) = 1.75" (1.75 inches high from top to bottom)) 2U = 3.5" 3U = 5.25" 4U = 7"
The number of rack units in a 'full cabinet' varies between providers - but
typically expect a full cabinet to be in the 40s Rack Units.
Some data centers have cabinets standing at over 7 feet tall (52 U +).