Each "U" space measures 1.75 inches (44.45 mm) in height.
The fundamental unit of vertical height is the (or RU). Eia Eca-310-e Pdf
The standard defines several critical measurements that are universal to "EIA-compliant" equipment: Rack Unit (RU or U) : Standardized as exactly 1.75 inches (44.45 mm) in height. Vertical Hole Spacing Each "U" space measures 1
The distance between the vertical rows of mounting holes is set at 18.312 inches (465.1 mm). Eia Eca-310-e Pdf
. If you measure from the center of the top hole to the center of the fourth hole down, it should be exactly 1.75 inches you need for a specific hardware setup?