Explaining Server Rack Dimensions

Server racks come in a huge range of sizes, so it’s easy to become overwhelmed by the options when you first start thinking about how to build a server. However, there are a few general rules that you can use to decide on the best server rack sizes for your requirements. 

Start by thinking about how you plan to use your servers. Then move on to looking into the different options that are currently available. Once you know how wide, high and deep your servers and related accessories will be, you’ll be able to choose a suitable server rack that does everything you need it to do. 

Whether you’re thinking about ways to build the server your business needs for its grand expansion plans, or you’re looking for a more understated option to suit your home or small business needs, there are plenty of great options for you to choose from. 

When we think about server rack dimensions, we are focusing on three elements – width, height and depth. The following guide details out the most important considerations for each of these dimensions, providing you with the understanding you need to make the best decision for your specific rack requirements.

What is a server rack? 

A server rack is a structure designed to hold technical equipment. This equipment can include the following: 

• Routers
• Switches
• Hubs
• Servers. 

Server racks are used by businesses or data centers to hold multiple pieces of equipment in one area. 

How wide are server racks? 

It’s often stated that the standard server racks are 19 inches wide. This is because the vast majority of rack-mounted equipment will have a mounting width of 19 inches, so keeping the width of server racks the same makes life easier for everyone. The technology will fit a 19-inch server rack perfectly, so there’s no need to worry about measuring individual pieces of equipment.

The average exterior width of 19-inch racks is then considered to be around 24 inches. The 24-inch width works well for data centers, as their floor panels are usually 48 inches. So, floor space can be used very efficiently with racks of this size. When server racks with a 24-inch exterior width are used, there’s still plenty of room to add cables and other equipment.

At TRG, we tend to work with rack and cabinet widths of 24 and 30 inches as these allow for more effective cable management setups – but how do you know which width will be the most useful for you?

24-inch width vs 30-inch width

The main considerations here are the use of massive cable trunking and the heaviness of applications.

A 30-inch wide rack ultimately allows for more extravagant amounts of cable management and fiber troughs running the vertical length of the rack on both sides. This empowers you to do highly dense network aggregation.

We would thus recommend this width if you are trunking a very large amount of cables into a particular rack, particularly on the order of low 100s of cables. This would therefore be useful for MMR aggregation racks, network aggregation racks, or centralized storage aggregation. If this doesn’t apply to you, then 24-inch wide racks, with the right depth, can more than get the job done if you have the right cabling policy in place.

Lastly, the final consideration is how heavy your applications are. If you have very heavy applications, or have something like 48U racks, then having a wider rack (30 inches) allows for less of a cantilever at the top, providing a more stable base and capability.

This wider rack also provides the opportunity to spread the load out on your raised floor, which is also good for weight management.

Takeaway:

The main rack width question to ask yourself: Do I have extraordinarily heavy racks or racks with extreme trunking/cables?

If yes, then the answer is to go for a 30-inch wide. If not, then 24 inches will likely be more than suitable.

What do I need to know about server rack height? 

If you are looking to increase your density or add more devices into a rack, or free up more space for trunking and cable management, then rack height is a useful consideration.

Server rack height is determined by something referred to as the U count. The ‘U’ in this abbreviation means rack units, and these have a standard height of 1.75 inches. Over time, 42U, 45U and 48 U racks have become relatively commonplace in data centers, as they provide plenty of space, while still keeping the size of the rack small enough that it can be managed and maintained with ease.

As a side note, you need to be careful about loading taller racks. The crucial thing to remember is that you should load from the bottom up, not the top down, in order to provide a stable base.

But how can you determine which U-count is the right one for your needs?

The right U count

As mentioned above, the garden variety of racks come in 42U, 45U, and 48U and more U space = a taller rack.

The main consideration here is more practical. Door clearance height is a sometimes overlooked factor, but it’s incredibly important. Many legacy facilities and offices can’t fit 45U and 48U racks – so remember to check the heights of your doors before attempting to use these.

With additional U space per rack also comes additional potential devices/density. Consider the port count for your PDUs to make sure you have additional power ports in your power strips to support the additional space.

Takeaway:

Main server rack height question(s) to ask yourself: Do I need additional U space for trunking, patch panels, or additional devices and can my facility support this?

Half Racks

There are a few other height options that you may come into contact with while researching server rack dimensions. These include 22U and 27U racks, otherwise known as a half-rack. Some non-standard sizes are also available, and you’ll notice these in abundance if you’re looking at open-frame racks. Open-frame racks come in quite a wide variety of heights, ranging right up to 70U for the very highest options.

Simply put, half racks are the most useful if you don’t need the space of a full-sized rack. The main consideration for half rack (21U) vs full-sized rack (42U) goes along with the main height question stated above.

Server rack depth: How much does this vary

Most often measured in inches, the depth of a server rack can theoretically fall anywhere between 1 and 50 inches depending on requirements. It is most common to see depths around the 42 inches (1070 mm) or 48 inches (1200mm) mark, though there are some exceptions.

For instance, brands like Dell, IBM and HP have been known to use shallower 29-inch racks for open frames.

There is some flexibility when it comes to pairing the right depth of rack with your chosen server. If you have a rack with a considerable depth, you’ll be able to make your own adjustments using rails, so you may be able to accommodate a smaller size server if you need to.

Here at TRG, we mainly work with server rack depths of 42” and 48”. The question is, when is each depth more appropriate?

The practical considerations of 42 inches vs 48 inches

The main considerations for success with server rack depth are if you can have low-profile cable management and if you have the right equipment (because some servers need deeper racks).

42” is considered the gold standard for data centers. For this depth, it’s important to take extra care with the cable management in the back. This is due to the limited space for both PDUs and effective cable management. When this is the case, we’d recommend utlizing low profile cable management. Alternatively, a better option could be to trust the cable management to a professional – which services like Colo+ allow.

Be aware that some servers will be too deep to fit in a 42-inch rack – this is commonly seen with large storage appliances such as EMC Isilon which have extended backs and thus won’t fit.

In other words, be careful about the type of rack you choose vs the size of the appliances you have. Larger equipment will likely require more rack depth, so 48 inches would be the better option and vice versa.

As a side note, if you do find yourself stuck in a particular cabinet with a longer server it may be possible (in a secure space) to take the doors off the back of the cabinet and allow the device to slightly protrude.

Takeaway:

Main server rack depth question to ask yourself: Do I have deep equipment?

If your equipment is known to have longer backs, then a deeper 48-inch rack is the better solution.

How to load the rack 

Physical dimensions have a part to play on the loading of the rack. Consider the weight of equipment and think about the physical size, in particular its width, to check its suitability. 

Cantilever racks are designed to accommodate heavy equipment, but we would always advise the heaviest equipment is placed as far down in the rack as possible. Heavier equipment placed higher up would require a wider rack, to balance the force of the weight on the rack itself. 

Think about your cable management strategy when deciding on the width of your rack. Wider 30 inch racks are ideal for systems requiring a great deal of cable management space. Ultra high density network cabling, for example, typically requires a rack of this width. 

The width of the rack is also important in terms of raw space planning purposes. Wider racks also provide more options in terms of what you can put into the racks. A 24 inch rack, in comparison, is far less forgiving. And that means that to use one of these you’ll need the right cable management strategy from day one – or you’ll need to use a fully managed service like Colo+. 

Determining how many devices will fit

In simple terms, the taller the rack is, the more devices you’ll be able to fit. A greater height provides more U space. A 48 U vs a 42 U rack will accommodate a far greater number of devices than a typical setup. Standard racks are 42 U; the largest we’ve seen so far is in the region of 52. 

There’s no right or wrong answer as to the correct height of a rack. It depends entirely on a company’s requirements in terms of its applications and whether it’s housing the right number of devices in its rack, relative to overall goals. In some cases, a full height rack is surplus to requirements, and a half rack is perfectly adequate. Think about what your company really needs to make sure your setup is sized correctly, and cost-effective. 

How much room should you leave? 

This is a question that we’re often asked, and it’s one that no one has a definitive answer to. Often, businesses will opt to leave space for hardware refreshers, and in some cases an adjacent rack will be left empty to enable teams to leapfrog around. Think about the size of your largest device, and consider whether you have the amount of space that this device requires free. 

Leaving 4U open typically allows for teams to swap one one device in a cluster at a time, which is helpful in some setups. Similarly, if you’re dealing with a lot of cross connects or interconnections, we’d advise you to think about how much room you need – and then double it. 

Again, there’s no right or wrong answer to this one – it really depends on what you need now and what you might need in the near future. But it’s well worth discussing this with your data center team if you need more clarity on the issue. 

Rail compatibility issues to consider  

This is a simple one – but it’s one that you overlook at your peril! When we talk about rail compatibility, we really mean circular vs square holes. We rarely see circular holes these days – square holes are far more common. 

Go back to the diagrams showing the server rack dimensions and look at the rails. Usually these are adjustable, but you’ll want to check the spacing on holes here. We have seen fixed, non-adjustable rails with issues with internal sliders, even when making rail adjustments.

In practice, the vast majority of servers do come with adjustable rails, so there’s a degree of flexibility there. But it’s something that’s worth checking at an early stage. 

Fitting towers in rack

Towers will fit in standard racks. There’s no need to invest in wider racks to accommodate towers. Most towers will fit in on their side, and positioned in this way they’re the perfect dimensions for a rack-mountable device. 

Make sure you have the appropriate shelving, and you should be able to fit between six and eight towers in a rack. 

Which devices require a deeper rack?

Heavy duty storage devices tend to require a deeper rack. Seagate storage devices, for example, deeply stack disks on a long chassis. Doing so provides more storage in a single rack. We have fit these in 42 inch racks in the past, but it means that PDUs have to be moved to the edge. In some cases, people choose to remove the back doors in order to fit such devices but this is obviously not ideal. 

Storage devices tend to drive requirements in terms of the physical dimensions of a rack, so it’s important to check that these devices are compatible with your rack, or if you might need to upgrade to a 48 inch depth rack. 

Are there any racks that should be avoided? 

There are no racks that we’d tell you to stay away from. What we would say is that you need to ensure that cable management is compatible with your rack. 

In our experience, people tend to underestimate how different cable management solutions can be. This means that issues like mounting aren’t properly thought through, which can lead to problems further down the line. 

Of course, you’ll want to use reputable racks. And the importance of this grows in line with the weight of the equipment you’re using. Opt for robust racks that are proven to be capable of accommodating everything you need. 

What you need to know about internal server dimensions

Once exterior dimensions like width, depth and height have been considered, we can begin to think about what’s known as the tertiary dimensions. These are things like interior rail dimensions and dimensions of the cable bars, for example. Other important tertiary dimensions include the spacing on the horizontal sliding rail for internal rails. 

The good news with internal server rack dimensions is that most racks will be able to handle it from a rail accommodation perspective. The devil’s in the detail around cable management, weight distribution, and access as mentioned above – which is why different external cabinet sizes are offered.

Internal rails have a standard depth, but they’re also adjustable. However, the dimensions of the cable tray should be given full consideration. Think about things like the dimensions you need for the cable tray, and the spacing required for holes where cables will go in and out. 

Consider whether you’ll need holes beneath the rack for power cables to be brought in, or whether power will be fed through the top. These are just a few examples of the finer points of server dimensions that are worth discussing as you plan your setup. 

Rails can also be a challenge and drop risk because of how different they all are. TRG uses electric server lifts to ensure things aren’t dropped when added or removed – and we also use custom-designed universal rails that allow for us to have a consistent racking experience. To learn more about our managed colocation offer visit our Colo+ page.

How much space should there be between server racks? 

Space is a key consideration when planning any server setup, and it’s something that is well worth bearing in mind while you choose your server rack dimensions too.

Whether you’re using a data center or your own space, you’ll need to make sure you’ve got room not just for your server racks, but for the right amount of space between and around them too. In general we want to see a minimum of 30-inch hot aisles for clearance – but the ideal space is 36-inches.

For cold aisles, we find 4 ft provides adequate space to allow you to properly remove equipment. The thing to remember is that cabinets may be placed directly next to each other, so the main consideration really has to do with the access aisles.

Aisle pace isn’t the only thing you’ll need to think about at this point, though. Other key considerations include the placement of units that may affect the server, such as any heating or air conditioning units. Server racks must be far enough away from these to avoid any damage that could be caused by drips or leaks. Bear in mind the thermal requirements of your server too, and think about both security and future proofing before you make your choice.

Get advice on server rack dimensions

Server racks are an important piece of any server setup. Their dimensions can have a real impact on how your system works, and what it’s ultimately capable of doing. 

If you have questions about server rack dimensions, or you’re unsure about the next step for your business, we’re here to help. 

At TRG Datacenters, we build the best data center experiences. Our Houston Data Center is built for fault tolerance, meaning we can focus on providing the best experience for our customers. 

Is it time to find out more? Get in touch and a member of our team will get back to you with more information.