A Guide to Server Rack Sizes for Data Centers

This guide deciphers the best server rack sizes for optimal data center efficiency. Learn why it matters, what options exist, and how to pick the perfect fit for your workloads.

Christopher Tozzi, Technology Analyst

January 8, 2024

5 Min Read
Server rack sizes for data centers, an in-depth guide
Andia / Alamy

What's the best server rack size for your data center? That's a simple question with a complicated answer. Today, server racks are available in a wide range of sizes, each with different pros and cons. Businesses must consider a variety of factors when selecting the right server rack size to fit their needs.

With this reality in mind, keep reading for a guide to server rack sizes, including why server rack size matters, which sizes are available, and how to choose the right rack size for your workloads.

What Is Server Rack Size, and Why Does It Matter?

Server rack size – also known as cabinet size – refers to the total size of the racks that house servers in a data center or other hosting facility.

Rack size is important because it determines how many servers you can fit inside each rack, as well as which types of servers the rack can accommodate. In addition, rack size can affect the amount of cables and power equipment you can stuff inside a rack, which may be important in situations where you require extra cables.

Understanding Server Rack Size

Server rack size is determined by three basic dimensions:

  • Height: Since servers are usually stacked on top of each other inside a rack, rack height is the single most important factor in determining how many servers you can fit inside a cabinet. Server racks are typically around 7ft (2.1m) high, but taller options are available if you need to accommodate more servers.

  • Depth: Server rack depth, meaning the distance between the front and back of the cabinet, can affect how large each server can be. Most servers don't exceed depths of about 42 inches (around one meter), but some require some extra space in the rear of your rack.

  • Width: The width of the rack from side to side is generally less important than rack height and depth because servers rarely exceed the standard width of 19 inches (48cm). However, in some cases, you might want an extra-wide rack to provide ample space for cables and power

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What Is the Standard Size for Server Cabinets?

There is no official standard size for server racks because no industry group or regulatory agency has deemed any specific dimensions to be standard.

However, the most common server rack size you'll encounter in data centers is 42U. A 42U rack is one designed to accommodate 42 servers (the "U" stands for "units," which in this case refers to servers).

That said, calling 42U racks standard is a bit misleading because it's really just the server height that is consistent. Most 42U racks are about 73.5 inches high because that's how much height is required to accommodate 42 servers that each require 1.75 inches of vertical space. However, the width and depth of 42U racks can vary. 42U cabinets are usually 19 inches wide and 40 or 42 inches deep, but there are plenty of exceptions.

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On top of this, it's worth noting that there are plenty of other rack sizes besides 42U that are relatively common. 48U racks, which provide a bit more capacity than 42U cabinets, are another popular choice in data centers, and much smaller rack sizes (such as 6U and 4U options) are also mass-produced for use cases that involve smaller numbers of servers.

The bottom line here is that while server racks designed to accommodate 42 servers are the most common type of rack you'll find in data centers today, there is really nothing approaching a true standard or norm for exact rack dimensions.

Choosing the Right Server Rack Size

Given the wide variation in server rack sizes, how do you decide which rack size is best for your needs? There is no easy answer, but in general, you'll want to consider the following factors:

  • Server room height: Taller server racks are more economical because they allow for a higher density of servers relative to the square footage of your server room. However, you'll want to make sure you leave enough airspace above the racks to allow air to circulate for cooling purposes, and to accommodate any cables or ductwork that needs to exist above your servers. So, while taller is usually better, you need to work within the vertical limits of your data center facility.

  • Cable space requirements: As mentioned above, servers that require more cables will benefit from racks that offer greater depth.

  • Cooling needs: The way you cool servers can impact rack size. If you rely on air to dissipate heat, you'll want to ensure that your rack is not so cramped that air can't circulate. But you also don't want to leave excess empty space inside the rack, which will increase the amount of air required to cool the servers. These considerations don't apply if you opt for other cooling methods, like liquid cooling.

  • Scalability requirements: If you expect to add servers over time, you will of course want to choose a rack size that gives you room to grow. If you don't anticipate expansion, you can save money by purchasing a smaller rack.

  • Rack layout: Rack depth and width can impact how you lay out rows of racks on your server room floor, which can in turn affect the energy efficiency of your infrastructure. Avoid choosing racks that are too wide or deep if doing so will translate to a less efficient server room layout.

Rack Size Matters

Server rack size may seem relatively simple at first glance, but don't assume that 42U racks are the way to go just because they're the most popular.

Instead, assess your overall priorities, such as how many servers you need to deploy today and in the future and your energy-efficiency requirements, then select a rack size that caters to them.

About the Author(s)

Christopher Tozzi

Technology Analyst, Fixate.IO

Christopher Tozzi is a technology analyst with subject matter expertise in cloud computing, application development, open source software, virtualization, containers and more. He also lectures at a major university in the Albany, New York, area. His book, “For Fun and Profit: A History of the Free and Open Source Software Revolution,” was published by MIT Press.

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