Data center tiers form an ascending classification system, ranging from 1, 2, 3, and 4, with some operators advocating for an even higher tier 5 level. This system is utilized to consistently assess data center facilities based on their potential site infrastructure availability, commonly referred to as uptime. Specifically, these tier ratings define the levels of redundancy and resiliency a data center can provide, as well as the estimated downtime a customer might encounter annually.

As a general rule, the difference between data center tiers is that tier 1 offers no redundancy of any critical system, tier 2 has partial redundancy in their electrical & HVAC systems, tier 3 contains dual redundancy for power & cooling equipment, and tier 4 possesses fully redundant infrastructure.

Understanding Data Center Tiers

Data centers are commonly rated by the Uptime Institute, an independent organization, which has issued over 3,400 certifications to data centers in more than 114 countries. The Uptime Institute ranks data centers through four distinct tier certification levels: Tier I, Tier II, Tier III, and Tier IV. In terms of availability, Tier I has the most expected downtime or worst performance, while Tier IV offers the least anticipated downtime or best performance.

Data Center Tier Classification System

Importantly, the Uptime Institute’s four data center tiers are progressive, meaning each level includes the requirements of all the lower tiers.

What Factors Determine a Data Center’s Tier?

The Uptime Institute’s tier certification assigns ratings to data centers based on criteria for power, cooling, maintenance protocols, redundancy (i.e., duplicating critical components), and fault tolerance.

At the same time, these ratings are agnostic to the specific technologies and vendors that are supplying equipment to the facilities. Therefore, data centers are assessed purely on their capabilities and not on which manufacturer they source their electrical systems and HVAC components from (e.g., Vertiv, Schneider Electric, and Eaton).

How Do Data Centers Obtain a Tier Rating?

Data center tier certification ratings are not a regulatory requirement, meaning they are optional. Therefore, not all data centers have an assigned tier.

To attain a tier certification from the Uptime Institute for a data center facility, an operator is required to submit its site plans along with engineering and architectural specifications, referred to as the Tier Certification of Design Documents, for review by the Uptime Institute. Following this, the Uptime Institute’s team conducts an on-site inspection of the data center to evaluate its operations and ensure alignment with the submitted design documentation. After completing the review, the Uptime Institute then awards a corresponding rating to the facility.

Data Center Tiers – What’s the Difference Between 1, 2, 3, 4, and 5?

Data center tiers, as classified by the Uptime Institute, include the levels of 1, 2, 3, and 4, while data center operator Switch Inc offers a proprietary tier 5 standard.

Note: For simplicity, this article refers to data center tiers using their numerical values, instead of the Roman numeral syntax used by the Uptime Institute.

CategoryTier 1Tier 2Tier 3Tier 4Tier 5
Uptime (Minimum)99.671%99.741%99.982%99.995%99.999%
Downtime (Annual)<28.8 hours<22.7 hours<1.6 hours<26.3 minutes<5.3 minutes
RedundancyNonePartialN+12N or 2N+12N or 2N+1
Concurrently MaintainableNoNoPartialFullFull
Cost to Build$$$$$$$$$$$$$$$

While data center redundancy and resiliency are of critical importance, it also worth noting that there is a direct correlation between the tier level and the cost to build a data center.

Tier 1 Data Center – Basic Capacity

Tier 1 Facility with a Server Rack in Compact Environment with Overhead Cables

Tier 1 data centers provide dedicated space for IT infrastructure to support workloads beyond an office environment. This IT infrastructure includes an electrical backup generator, uninterruptible power supply (UPS), and HVAC / cooling systems, such as a computer room air conditioning (CRAC) unit. Notably, these facilities have a single distribution path for power and cooling infrastructure, and there is no redundancy of any critical system.

Collectively, these basic data center components enable the facility to manage through power spikes and outages, as well as operate outside of typical office hours – meaning nights and weekends. However, in order to perform regular maintenance or emergency repairs, the data center operations must be shut down temporarily.

Given a tier 1 data center’s limitations, they are best suited for very small enterprises seeking the most cost-effective hosting solution. Additionally, these organizations need to be able to tolerate meaningful periods of downtime over the course of a year (i.e., up to 28.8 hours annually).

Tier 2 Data Center – Redundant Capacity

Tier 2 Data Center Building with a Glass Windows set Alongside Roadway

Tier 2 data centers contain all of the capabilities of tier 1 facilities, but also include partial redundancy in the form of electrical backup generators, power components (e.g., UPS systems), and cooling equipment (e.g., chillers / pumps). This redundant infrastructure enables tier 2 data centers to undertake select maintenance opportunities, while providing an increased level of protection against IT process disruptions resulting from equipment failures.

Similar to tier 1 facilities, tier 2 data centers rely on a single distribution path for power and cooling, which makes these facilities vulnerable to unexpected disruptions.

Tier 2 data centers are commonly used by small- to medium-sized enterprises that want a cost-effective and more reliable option for their compute and storage needs. Typically, these organizations use tier 2 facilities for data backup and recovery purposes, as well as non-mission critical functions.

Additionally, tier 2 data centers satisfy the requirements of customers who have connected together their data centers or can afford a reasonable amount of downtime (i.e., up to 22.7 hours annually). Frequently, a tier 2-rated data center is designed close to tier 3 standards, however, the operator may have chosen to reduce costs by fitting-out the facility at tier 2 levels for non-critical components.

Tier 3 Data Center – Concurrently Maintainable

Tier 3 Data Center Large Information Storage Facility with Rows of Server Racks and Cabinets

Tier 3 data centers contain all of the capabilities of tier 1 and 2 facilities but do not require shutdowns during planned maintenance and equipment replacement, hence they are concurrently maintainable. To implement this resiliency, an additional redundant distribution path for power and cooling is added to the existing redundant critical components of tier 2 facilities so that all components needed to support the IT processing environment can be shut down and maintained, without impacting IT operations.

More specifically, all IT equipment must have dual power supplies attached to different UPS units, ensuring that a UPS unit can be taken offline, without causing servers to crash or creating a loss in network connectivity. Also, redundant cooling systems must be in-place such that if one cooling unit fails, the other one starts-up and continues to moderate the temperature and humidity in a data center’s computer rooms.

By being concurrently maintainable, a tier 3 data center has systems, protocols, and equipment in place to permit staff to perform maintenance and repairs without interrupting services to customers.

N+1 Redundancy

Tier 3 data centers offer more comprehensive redundancy capabilities and a significant improvement in availability. These facilities feature N+1 redundancy, which means that for every essential component, there is at least one additional backup component. This backup component is ready to take over in case of a single failure or during planned maintenance of the primary component. Below are the definitions of N+1 redundancy:

  • N: necessary capacity in order to support the full IT load of the data center
  • +1: extra component for backup purposes

Also, a tier 3 data center requires backup solutions which can keep operations running, for at least 72 hours, following a local or region-wide power outage.

Not Fault Tolerant

Tier 3 data centers are not entirely fault tolerant because they often rely on, or share components, which are not completely independent to the facility. For example, these components could include utility company power feeds and external cooling systems, such as chillers and cooling towers, that reside outside the data center.

Industry Standard

Tier 3 data centers are the industry standard, providing customers with high-quality space, power, and cooling, which delivers a sufficient level of redundancy and resiliency to support a customer’s mission-critical compute and storage needs. As such, tier 3 data centers are preferred by larger enterprises, particularly if they host and manage customer data.

Tier 4 Data Center – Fault Tolerant

Tier 4 Computer Hub Multi-Story Building with Security Gate at Entrance

Tier 4 data centers contain all of the capabilities of tier 1, 2, and 3 facilities, but also include fault tolerance mechanisms, with redundancy for every component. Specifically, fault tolerance means that when any unplanned individual equipment failures or distribution path interruptions occur, the effects of the events do not impact IT operations.

Essentially, tier 4 facilities have no single points of failure. Therefore, the objective of a tier 4 data center is to provide continuous support and services no matter the circumstances.

2N or 2N+1 Redundancy

Tier 4 data centers are distinguished from Tier 3 facilities primarily by their redundancy levels, featuring either 2N or 2N+1 redundancy. This translates to fully redundant infrastructure. Specifically, 2N redundancy implies that for every component, there is an entirely identical backup system in place. Crucially, this backup system is physically separate and independent from the primary system. Such physical separation is key in ensuring that a local event does not impact both systems simultaneously.

More specifically, all components are supported by two different utility power suppliers, two electrical backup generators, two UPS systems, two power distribution units (PDUs), and two different cooling systems.

Furthermore, each distribution path is independent, such that a single failure in one does not cause a chain reaction of failures with other components. If any individual power or cooling infrastructure component fails in a tier 4 data center, IT processing will continue without issue. Therefore, IT processing can only be impacted if components from two different electrical or cooling paths fail.

Also, tier 4 data centers require backup solutions which can keep operations running, for at least 96 hours, following a local or region-wide power outage. Finally, the facility’s power source does not connect to any external source, meaning it must be independent.

Cost to Build

Given the fully redundant requirements of a tier 4 data center, the costs of fitting-out such a facility are much higher and, as a result, tier 4 data centers remain relatively rare. For example, the cost of constructing and fitting-out a tier 4 data center can be 25% to 40% more than a tier 3 data center and double that of a tier 2 data center.


Tier 4 data centers serve the needs of enterprises who are not price-sensitive and require uninterrupted availability. For example, large enterprises (e.g., financial services firms) with mission-critical workloads and demanding customer or business needs are often users of tier 4 data centers.

Tier 5 Data Center – Fault Sustainable

Tier 5 Data Center Servers from Switch Inc a Colocation Operator
Source: Switch Inc.

Tier 5 data centers go beyond the Uptime Institute’s tier certification standards. Specifically, Switch Inc, a colocation provider, introduced a proprietary Tier 5 Platinum standard, which attempts to set a level above fully redundant systems and total fault tolerance. This standard incorporates more than 30 additional elements critical to data center design and constant operation.

Switch’s Tier 5 Platinum standard includes rigorous parameters regarding long-term power system capabilities, the number of available on-net carriers, the durability of roof systems, the location of cooling system lines in or above the data center, physical & network security, and 100% use of renewable energy.

Why Do Data Centers Get Tier Certifications?

Data Center Tiers Tier 3 4 5

The Uptime Institute’s tier certifications provide a quick reference point to determine a data center’s level of performance. For data center operators, these ratings often deliver benefits in the following ways:

  • Marketing: Operators can show existing and prospective customers that their data center meets credible standards and can support the needs of enterprises, as determined by an independent third-party
  • Customers: Data centers rated tier 3 or higher may meet the criteria of a wider audience of prospective customers, who internally set minimum certification requirements. This is particularly important for newer data center infrastructure operators, which do not have a pre-existing relationship with a particular enterprise
  • Improvement: Certification assessments offer guidance on addressing deficiencies, with guidelines for making future improvements and upgrades to the data center’s design, construction, and operational practices. This process helps identify areas for improvement in energy efficiency, leading to better resource management and cost savings

Data center tiering classifications serve as a crucial factor in assessing a data center’s quality. Customers, especially those with specific operational and availability needs, rely on these classifications to gauge a facility’s suitability to meet their business requirements. This is particularly true for larger enterprises, including multinational corporations and telecommunications carriers, who have their own set of rigorous standards and criteria for evaluating data centers.

Data center tier certification ratings, while not mandated by regulations, play a crucial role in shaping the design of a data center. Typically, the specific design of a facility is dictated by the commercial agreement between the data center operator and its customers, catering to the unique needs of each customer. Consequently, many data centers opt not to undergo a formal tier certification process.

Despite this, the importance of data center tier certifications remains significant. They offer a universally acknowledged standard that targets reliability and performance benchmarks, which are important in securing customer trust and satisfaction.

Mary Zhang covers Data Centers for Dgtl Infra, including Equinix (NASDAQ: EQIX), Digital Realty (NYSE: DLR), CyrusOne, CoreSite Realty, QTS Realty, Switch Inc, Iron Mountain (NYSE: IRM), Cyxtera (NASDAQ: CYXT), and many more. Within Data Centers, Mary focuses on the sub-sectors of hyperscale, enterprise / colocation, cloud service providers, and edge computing. Mary has over 5 years of experience in research and writing for Data Centers.


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