To calculate rack units for your equipment, you need to know the height of the device you plan to mount. The standard 1U rack size equals 1. This article explains definition, planning, installation tips, and trends. [][] 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. U (rack unit, RU) is a unit of equipment height in a 19" rack. 45 mm), defined by the EIA-310. Measure your deepest server and add 3–6 inches for cabling and airflow. Use the. For this, a special unit of measurement, U, is used.