What Is Data Center U Space? Key Considerations for Choosing the Right Cabinets and PDUs

May 20, 2025

Sometimes we get so used to abbreviations and acronyms that we forget what they even stand for. So, let’s dive a little deeper into U, the unit of measurement that guides data center personnel in calculating how many devices fit in a rack. 

U or RU stands for “rack unit,” a standardized method to measure mountable height in a server rack. One rack unit equals 1.75 inches. To estimate a rack unit, look at the vertically arranged threaded, square, or circular holes on a rack. Three of these holes equal 1.75 inches, or one U. The holes are used to mount and secure equipment. 

U Space: At-a-glance guidelines  

Rack manufacturers adhere to the Electronic Industries Association (EIA) standards, ensuring that mounting holes are consistently spaced. This eliminates the hassle of trying to fit equipment into differently sized racks. Racks are essential for enabling space optimization when housing multiple generations of IT equipment. A standard rack measures about 42U (19 inches), with servers stacked atop one another to maximize space. Typically, servers occupy 1U of space, though some come in 2U or 4U sizes. Larger hardware may require more space – as much as 12U. 

While racks follow standardized U heights, the depths, and sizes differ slightly by manufacturer. The industry standard is 19 inches wide and 36 or 42 inches deep. "Full rack" server models are 19 inches, while “half rack” equipment, usually networking gear, takes up 9.5 inches. Common heights are 42U (or 73.5 inches) or 48U (84 inches) for higher-density applications. 

PDUs: Vertical or Horizontal, and what is Zero U? 

Besides U sizes, another consideration for planning data center space is whether to deploy horizontal or vertical rack power distribution units (PDUs). While some data center administrators prefer one over the other, the choice comes down to specific needs, including factors like airflow and whether you have shallow gear or need to distribute power from in-rack UPSs.  

Horizontal PDUs, also known as “rackmount PDUs,” are mounted horizontally inside a rack, similar to a server, and typically have a 19-inch width. These units occupy 1U or 2U of rack space, so they need to be taken into consideration when planning the cabinet space. Will there be enough room for horizontal PDUs once you install servers and networking gear? 

If the answer is no, you have another option—vertical PDUs, also known as "rack PDUs." As the name implies, these are mounted vertically to vertical posts in a rack. "Zero" U refers to no units of rack space, which doesn't interfere with any of the gear mounted within the rack. Vertical PDUs are typically designed with a low profile to facilitate access to devices on the rack and allow for efficient airflow. 

Choosing the Right Data Center U Space 

Understanding what U space represents in the data center, whether it's referring to a cabinet or a PDU, will help you choose racks and equipment to populate your data center aisles and accommodate your needs. Legrand can assist with U calculations, helping to maximize rack space and ensure all hardware and components fit together.  

To learn more about Legrand’s cabinet options, click here. To explore our rack PDUs, click here.