What is a Rack Unit (RU)?

What Is a Rack Unit (RU)?

When you’re building up networking or server equipment, it’s common to mount everything on racks. The racks seem like they have the same sizing and hole placements, and that’s because these things are actually standardized across industries.

Because of these standards, many things are measured in “U.” For instance, you might see a 2U server. To understand exactly what that means and what you can do with this information, it’s important to explore the concept of a rack unit.

What is a Rack Unit? Where does it come from? You’ll see detailed answers below.

The Exact Rack Unit

Since a rack unit is a specific measurement, every rack unit is exactly 1 ¾ inches (which is 44.45mm). That never changes whether you are measuring vertically or horizontally.

Most commonly, RUs are used to describe racks (and equipment that goes in racks) on 19-inch frames, although the system also works with 23-inch frames. Understanding this, a full-size cage is 42U high, and equipment is usually anywhere from 1 to 4 units high (although some equipment is larger).

International Standards

The official rack unit comes from the Electronics Industries Alliance. This group came together to figure out ways to standardize the mounting strategies for any equipment that would go on a rack, and the solution they came to is known as EIA-310. This set of standards determines the official rack unit. It also defines the specifications for hole spacing — both vertical and horizontal.

EIA-310 is applied to racks, cabinets, and even a lot of audio equipment. If you see something measured in rack units, you already know exactly how it fits with standardized mounting equipment.

Once you understand the rack unit, you can apply that knowledge to design your infrastructure and know that everything will fit properly.

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