The Meaning & Significance of Rack Units (RU)

Server racks and cabinets have their own terminology and follow standards that may seem arbitrary at first. But a look under the hood can help you understand how these terms and standards developed.

What Does ‘RU’ Mean in Server Racks?

One of the fundamental concepts in server racks is the “rack unit,” abbreviated “RU” or just “U.” When you go to buy a rack or cabinet, its height will be specified by the number of U’s.

How Big Is a Rack Unit?

One rack unit (RU) is equal to 1.75 inches (44.45mm). The U standard is defined by the Electronics Industry Association in the EIA-310-D specification.

How Many Rack Units in a Standard Rack?

Rack height can vary greatly based on application and environment, but 42U is one of the most common heights on the market. This means that there are 42 rack units (73.5 inches) of usable space in the rack. It is important to note that the “RU” metric is used to measure usable space only. When you factor in the external height of a standard 42U rack, it will technically be taller than 42U.

Why Is a Rack Unit 1.75 Inches?

Some rackmount equipment has flanges or “ears” with round or square holes for attaching the device to the rack. Rackmount server rails also have brackets with holes that are similarly spaced. A rack unit is equivalent to three holes, which are spaced .625 inches apart on center, with .5 inches above and below. Both round and square holes are aligned in this fashion so that the three-hole group is always 1.75 inches.

Why Is the Rack Unit (RU) Metric Important?

The rack unit standard is essential to the data center and IT infrastructure industries. Data center managers can invest in racks and cabinets with the assurance that any manufacturer’s rack-mountable equipment will be measured by the same standardized height metric, RU. Additionally, the equipment’s mounting holes will always align with the holes in the server rack.

A 1U rackmount device will take up 1U in the rack, and you can mount pieces of equipment right above and below one another without leaving space in between. Most IT equipment takes in air at the front and exhausts heat out the back, so dense vertical installation will not cause overheating in most cases.

With that in mind, you can calculate what size rack you will need based upon the height of the equipment going into it. Most manufacturers will clearly specify the height of a server in U’s. With other types of equipment — routers, switches, uninterruptible power supplies (UPSs) and the like — you may have to dig a little deeper into the specifications (although most standardize on “RU”). Once you know the U specification of every device, you can add them up to determine the height of the rack.

Server Rack Height Variable to Consider

There are, of course, caveats. Not all equipment is meant to be mounted onto the rack’s vertical rails. Many routers and switches are installed on shelves within the rack or cabinet. Although server rack shelves are generally specified as 1U or 2U, the equipment sitting on them may vary in height. You may need to allow space above the shelf to access the equipment. These variables need to be considered in your rack height calculation.

Also, keyboards and monitors should be installed at a comfortable working height. You may have to “waste” some space to get things to work out correctly.

It’s a good idea to allow extra space for expansion. On the flip side, there are power capacity limitations that need to be considered as well. Building a high-performing, efficient, and easy-to-maintain server rack or cabinet can be a bit of a puzzle, but following best practices pays dividends in the long run. Check out our blog on server rack sizes and dimensions for more information about how to choose the right size.

Facility and Operational Considerations

When deciding on rack height, you also have to consider the logistics of the data center and IT operational efficiency. There are commercially available server racks that are 70U tall — just over 10 feet. Unless you have high ceilings in your data center, a 70U rack won’t fit or might interfere with sprinkler systems, ductwork, or other infrastructure.

Very tall racks can also create management challenges for data center personnel. Administrators will have to use a ladder to reach the top of the rack, which is time-consuming and hazardous. There will need to be enough headroom above the rack so administrators can work somewhat comfortably while standing on a ladder.

That’s why most data centers use 42U or 48U racks. By standardizing on this height, you can accommodate a lot of equipment while avoiding facility and operational problems.