What is height in U and how is it calculated?

The U (rack unitGlossary · UPS technologyRack Unit (U)A standard height unit for equipment in 19-inch racks, where 1U equals 44.45 mm.) is the standard height unit for rack equipment: 1U equals 1.75 inches, or 44.45 mm. A basic switch or patch panelGlossary · Communication and monitoringPatch PanelA rack panel with a row of ports where network cables terminate, so connections can be organised and changed easily. usually takes 1U, a server 1U or 2U, and a rackmount UPS often 2U or 3U, while external battery packs need extra U.

To calculate the height you need, add up the U of all the equipment, add space for cable organisers, shelves and horizontal power strips, and leave a margin for expansion. For example, a 1U switch, two 1U patch panels, a 2U shelf for the router, a 2U UPS and two 1U cable organisers come to 9U, so a 12U rack leaves a reasonable margin.

Note: the nominal height in U refers to the usable space on the rails, not the external height of the rack, which is greater because of the base, top and castors. A common mistake is counting only the equipment, without the cable organisers and room for airflow, which leaves you with a rack that is full from day one.