A rack unit, U or RU is a unit of measurement that describes the height of equipment designed to mount in a 19″ rack. Racks offer room for equipment that is 19″ wide, and have variable heights, which are expressed in rack units. One rack unit (1U) is 1.75″ (44.45 mm) of vertical space, or typically the equivalent of three rack hole spaces tall.
One of the first criteria to consider when purchasing a rack is how many RUs your equipment requires. Spacing is measured from center hole to center hole and does not vary, regardless of whether the rack angle has square or round mounting holes.