The rack unit (U) is a standard vertical measurement in server rack equipment. 75 inches (400 mm) high, making them optimal for compact environments like home labs, small offices, and edge deployments. How tall is a 9U cabinet? Wall-mount 9U cabinet internal/external dimensions, depth options and ideal use cases. [][] It is most frequently used as a measurement of the overall height of 19-inch and 23-inch rack frames, as well as the height of equipment that mounts in these frames, whereby the height of the frame or. The total height of a rack is calculated by multiplying the number of U (rack units) by 1. Height (in inches) = Rack Units (U) × 1.