rack unit
Define: Rack Unit âUâ or âRUâ. The hole spacing for standard 19â³ racks on the mounting flange is spaced in groups of three holes. This three hole group is defined as a Rack Unit (RU) or sometimes just call a âUâ. 1U occupies 1.75â³ (44.45 mm) of vertical space. Manufacturers of rackmount equipment make their product lines based upon how many RUs that the equipment occupies in the server rack.