Understanding U Space, and Why It’s Important

Data center racks are measured in rack units (RU), commonly abbreviated to U. One U measures 1.75 inches and is used to define the amount of vertical space available in a rack. Knowing what U stands for is critical for data center planning because it standardizes rack and gear height with a uniform measure.

The U measurement is a standard introduced by the Electronic Industries Association (EIA). The idea was to simplify rack planning for data center staff, regardless of which manufacturer provided the equipment. If you know a server occupies 2U of space, you know exactly how much room it will take up in the rack and how much will be left for other devices.

Everything Fits with U 

By looking at a rack, how do you know how many Us of equipment it can hold? Take a look at the vertically arranged threaded, square, or circular holes on a rack. Three holes add up to 1.75 inches, or one U. The holes are used to mount and secure equipment.

Evenly spacing the holes eliminates the hassle of trying to fit equipment into racks of varying sizes. Having a standardized metric is especially useful when placing multiple generations of IT equipment together in a rack.  

The typical rack measures 42U (19 inches), with servers stacked atop one another to maximize space. A server usually takes 1U of space, but some come in 2U or 4U sizes. Larger hardware may require more space – as much as 12U.

While racks follow standardized U heights, 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 – such as networking gear – occupies 9.5 inches. Common heights are 42U (or 73.5 inches) or 48U (84 inches) for higher-density applications.

Whether to Deploy Vertical or Horizontal PDUs 

U space is very important in rack planning. Another important consideration involves the type of rack power distribution units (PDUs) you’re going to deploy. PDUs come in vertical and horizontal formats, and there are practical considerations for deciding which to use, including airflow, whether the rack gear is shallow, and whether you need to distribute power from in-rack UPSs.

Horizontal PDUs, also known as “rackmount PDUs,” are installed the same way as servers – mounted horizontally inside a rack and typically measure 19 inches in width. These units occupy 1U or 2U of valuable rack space, so it’s important to ask: Is there enough room for them once the servers and networking gear are mounted?

If not, you need to install vertical PDUs, also called “rack PDUs”. Mounted along the rack’s vertical posts, they take up zero U, which means they don’t use any horizontal space inside the rack. Their slim profile supports better airflow and easier access to devices, making them a smart choice when rack space is tight. 

Choosing the Right Data Center U Space  

Standardizing rack measurement with U takes the guessing out of data center planning. Understanding U measurements makes it easier to choose cabinets, racks and hardware for your data center aisles. If you need assistance, Legrand helps you with U calculations so you can optimize your racks, ensuring all components are a good fit.

To learn more about Legrand’s cabinet options, click here. To explore our new generation PRO4X Rack PDUs, click here.