Define: Rack Unit “U” or “RU”


The hole spacing for standard 19″ racks on the mounting flange is spaced in groups of three holes. This three hole group is defined as a Rack Unit (RU) or sometimes just call a “U”. 1U occupies 1.75″ (44.45 mm) of vertical space. Manufacturers of rackmount equipment make their product lines based upon how many RUs that the equipment occupies in the server rack. The three hole group spacing is measured from center hole to center hole and does not vary, regardless of whether the server rack has square or round mounting holes. This uneven hole spacing can cause installation issues if the installer does not pay close attention to where the slide rail assemblies are being installed. The most common problem is that the slide rail assembly hooks or holes do not line up with the holes in the server rack. This is typically due to the slide rail assemblies being misaligned with the hole pattern or otherwise referred to as “being off a U”.



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5 Responses to “Define: Rack Unit “U” or “RU””

  1. Tom Rodby Says:

    You state “1U occupies 1.75″ (22.25mm) of”, the 22.25 is not correct and should be 44.5 mm.

  2. How tall can a server rack be? | The Server Rack FAQ Says:

    […] heights found in data centers are typically around the 42U to 44U range. By the way, one “U” or “RU” space is 1.75″ tall. So, by replacing a 42U with a 55U, you gain […]

  3. Universal Rails may be the answer | The Server Rack FAQ Says:

    […] as well. Frequently, you’ll find versions of Universal Rack Rails that support different U-heights and various weight capacities. Because the simple design doesn’t include rotating ears, […]

  4. Nate Says:

    There’s a Russian unit of measure called the “vershok” which is equal to 1.75 inches. So next time you’re ordering racks, specify their height in vershki!

  5. Gerardo Mertel Says:

    It is interesting to read your blog post and I am going to share it with my friends.republic