What is “U” height? Rack Units explained


When in the market for server racks, the most common differentiation between racks of the same ilk is how many “U‘s” they have. The U in this case refers to rack units, which is a standardized method of tracking mountable height in a server rack.

One rack unit is equal to 1.75 inches or three threaded, square or circle holes. These holes are placed in a vertical orientation and are used to secure mounting equipment to the rack.

How did rack units become a standard?

Like most standards in the server rack industry, rack units are a term promoted by the Electronic Industries Association (EIA). Without these standards in place, you might end up buying a rail that has differently spaced holes for threading than your rack.

Ultimately, it makes the user’s life easier because more of their equipment will be compatible with future equipment that they purchase. If it didn’t exist, manufactures might be more tempted to create proprietary standards to keep customers attached to their ecosystem.

The EIA-310 document is available for sale on the IHS market, but if you need measurements, you can check server or server rack manufacturers’ dimensional documents to get an idea. Since the widely used EIA-310-D iteration was published in 1992, it’s difficult to track down the root of how the term ‘rack unit’ originally gained popularity.

How many U (RU) spaces do I need?

On Dell’s website, every rack mount server starts with an RU classification. This should be the case for any reputable server manufacturer. If you are filling a rack with only servers, it will be easy to add up the rack units on your server and purchase something with an adequate amount of space.

Patch panels, switches and routers, on the other hand, are less likely to advertise a U height. This is because they aren’t typically mounted by rails that are more tailored for perfectly defined spacing.

You’re more likely to end up throwing a router onto a shelf, and not all shelves are sized the same. So, depending on the size of your shelf and the size of the networking equipment, you might not end up filling an exact 1.75 inches of space and will instead, need to calculate space used in more traditional methods.

Whatever kind of space you need, RackSolutions will have something that suits you perfectly. We manufacture server racks anywhere from 1U to 70U (sometimes even 90u). When narrowing down your server racks, don’t forget to make sure that the external height will work for you, not just the U spaces.

Dimensions of a rack unit

Rack U Spacing
Illustration of rack unit spacing

Though rack units are spaced out evenly, measurements of holes within them are a bit more. The total rack unit is measured from the middle of the rack’s top hole of the previous U space to the top hole of the current U space.

Spacing between holes within a rack unit are the same at .625 inches, but that on its own would only make up 1.35 inches of height. The remaining .5 inches comes from the distance from the bottom hole to the top hole of the rack unit below it.

These measurements do not change regardless of the hole being square, round or threaded.

What is "U" height? Rack Units explained
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What is "U" height? Rack Units explained
Rack units, otherwise referred to as "U" or "RU" is a standardized measurement of mountable height in a server rack or cabinet.
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Rack Units