A rack unit (RU) is the standard size for measuring equipment to be mounted in 19-inch racks used in almost all datacenters worldwide. Individual pieces of rack-mountable equipment such as servers, switches, routers, blades and so on can vary in size.
Racks typically accommodate equipment 19 inches wide with size variance being 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 to costs, 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.
Equipment are supplied power by means of Power Distribution Units (or PDUs). These units typically feature IEC or 240V General Purpose Outlets up to 10 Amps per point. 15 Amp outlets are available on request for high-power equipment.