Any furniture piece has dimensions. Server cabinets are specialized pieces of furniture for data centers. Thus, they also have dimensions. Moreover, dimensions play a crucial role in choosing appropriate models since they are designed to accommodate computing equipment. So, system administrators must make sure that particular models are spacious enough to house certain types of active hardware.

Traditionally, any furniture is measured in height, width, and depth. But, everything is not so easy with server racks. You’ve probably noticed the “U” letter in the names of server cabinet models. Do you have an idea of what is U in rack size? The “U” letter actually stands for height. In system administration, the height is measured in units (that is, “U”). 1U takes 1,75 inches.

How did Rack Unit Become a Standard?

This decision came as the second wave of global standardization. When IT technologies ceased to be the prerogative of a narrow circle of IT employees, it turned out that the only possible way to place the largest possible number of equipment units in a limited area is to place them “in a pile” and form a server racks.

This led to a reasonable requirement that the height of hardware blocks in the cabinet should be either of the same size or a multiple of some integer. The design is usually carried out in the form of 2- or 4-post server racks.

How are Racks Measured?


Since we’re done with server rack U meaning, we can proceed to consider how it is measured. Racks are vertical uprights with special holes made through a certain distance. Every three holes in the rack are exactly one unit.

The modern industry produces a wide diversity of models of different sizes and designs. Depending on the number of system components and available space, system administrators can pick an appropriate model for every data center.

Currently, the size of cabinets ranges from small 1U models up to huge 48U boxes. According to the cabinet size, experts classify server furniture into three major groups, namely:

  • Full Rack – between 42U and 48U;
  • Half Rack – typically 22U;
  • Quarter Rack – typically 11U.

Taking into account the statistics of sales, about 47% of customers prefer mid-sized models that can freely accommodate 2-10 servers and all the needed accessories. Compact 6U-9U models are common for small rooms and designed for fastening to walls. More spacious furniture pieces are chosen for large data centers. The most popular models are 22U, 32U, and 42U.

Helpful Tips to Pick an Appropriate Size in Us


When selecting a suitable rack size, system administrators should keep in mind the following criteria:

  • the number of servers you’re going to accommodate inside the cabinet;
  • other components that are 1,75-inch tall;
  • the smallest piece of hardware in U, which you’ll have to house;
  • equipment that is taller than 1U.

Judging by the obtained information, you’ll be able to calculate the required size of the cabinet. However, this is not the end. When picking server furniture, additional criteria play an important role. These are maintenance, security, expansion and power draw.

To make sure that the chosen model will serve your business for many years, keep in mind the following remarks:

  • System administrators always place equipment inside racks loosely. Tight fitting leads to hindered air circulation and overheating of the hardware.
  • It is recommended to add one-third of the required size as extra space for future system expansion. Your system is likely to grow. To avoid buying a new cabinet, purchase a bit larger model from the very beginning.
  • Tightly placed devices will harden maintenance. The staff will spend more time on repairs, which increases system downtime.

IT hardware is designed to store datum and valuable information. Its security is mainly determined by the proper fastening and allocation of devices on racks and in cabinets. The main concept to remember is that you should always leave free space in cabinets. Hardware is never placed tightly.

Now, you know the answer to the question “what does the U mean in server racks”. The U is an essential parameter in the IT sphere. System administrators should know how to measure devices to select a proper model for the system. Keep in mind that the smoothness of operation and its uninterrupted functioning highly depend on the correct choice of server furniture.