What is a rack unit or “RU”?
A rack unit, frequently abbreviated as “RU” or just “U”, is a standard size measurement for equipment to be mounted in 19-inch racks of the kind used in almost all data centres worldwide. Individual pieces of rack-mountable equipment such as servers, switches, routers, blades and so on can vary in size. Racks typically accomodate equipment 19 inches (482.6 mm) wide. So the size variance is sometimes a matter of depth; for example some simpler switches and routers are relatively shallow devices compared to a typical server, though are still the standard 19 inches wide. The main significant variance, however, is the height of the equipment and this is where the RU comes into play. The height of rack-mount equipment is standardised as multiples of 1.75 inches (4.445 cm) and this is what is known as a “rack unit”, “RU” or “U”.
