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”.
RU sizing and colocation pricing
The RU size of a server or other piece of equipment in a data centre is one of the factors affecting the cost, since a larger server will take up more rack space which is often at a premium. (The other main components of the cost of colocation are power and bandwidth.) Consequently, it is usually best to use the smallest possible server that meets your requirements. Often it makes financial sense to pay a premium for better, more compact servers due to the ongoing savings in rack space over the life of the server.
Sizing of equipment and racks
A server may be described as “1 RU” or “1 U” if it occupies a single rack unit. A full rack of equipment typically accomodates around 42 RU, heights vary between models and manufacturers and some racks may be somewhat shorter or taller than this.