Tier III vs Tier IV Data Center – What’s the Difference?

12 Dec 2014 by Datacenters.com Technology


What is the difference between a Tier III and Tier IV data center? Is it worth it to select a Tier IV facility? In this article we will be discussing the Uptime Institute, tier levels, and differentiating factors that will help you decide which tier data center is right for your IT infrastructure. 

The Uptime Institute is a leading provider of research and education for the enterprise data center industry and data center professionals.  Founded in 1993, Uptime Institute was the first to pioneer an end-user community and proprietary certifications focused on improving the reliability and uninterruptible availability in data centers and IT organizations. [1]

The company was later acquired in 2009 by The 451 Group, a technology industry analyst company focused on the business of enterprise IT innovation. Despite operating independently, the two companies work together to provide thought leadership, syndicated research, and professional services for the entire IT ecosystem covering facilities, hardware, and software. [1]


 The Uptime Institute categorizes data centers into four basic certification groups – Tier I, II, III, and IV. Each certification or tier represents different levels of availability, hours of interruption per year, and data center facility and system redundancy standards. [2]

Many data centers claim to be a certain tier level such as a Tier III facility. However, many not go through the certification process with the Uptime Institute even though they may have all of the necessary systems and procedures in place. Make sure to ask potential data centers for their Uptime Institute and other third party certifications. You will also want to confirm HIPAA and other regulatory and compliance standards.

Tier I 

Tier I certified data centers represent the lowest category awarded by the Uptime Institute. A Tier I data center represents an availability of 99.67% and has no built in redundancy. In fact, a Tier I data center can have up to 28.8 hours of interruption a year. This option might be sufficient for a small business, website, or blog hosting but certainly not for an enterprise with mission critical applications. [2]

Tier II 

Tier II certified data centers represents a slight improvement over a Tier I facility with 6.8 hours less of average downtime per year. Tier II data centers feature an availability of 99.75% with 22 hours of hours of interruption a year. These facilities feature partial redundancy to guard against downtime. [2]

Redundancy refers to the alternative systems such as power, HVAC, and network used in case of disruption as a failover on a temporary basis. The premise of redundancy is continuous operation of IT server hardware and related networking equipment.

Tier III 

Tier III certified data centers are often referred to as three 9s. Most enterprises select Tier III data centers for their uptime and redundancy measures. There is a huge jump in the availability of a Tier III data center opposed to a Tier II data center. A Tier III facility has availability of 99.982% with only 1.6 hours of interruption a year. [2]

A Tier III data center has N+1 Redundancy. N+1 means that the facility has what is required to operate plus a backup. Redundancy can include items like power feeds, diverse network paths, UPS, and diesel generators.

Tier IV 

The Tier IV certification is currently the highest classification among data center facilities. In fact, Tier IV facilities are the equivalent of data centers used by the United States Government. They are highly secure, reliable, and redundant.

Tier IV is sometimes referred to as the four 9s. Tier IV data centers have an availability of 99.995% and 0.8 hours of interruption per year compared to 1.6 hours of interruption with a Tier III facility. That is half of the downtime represented by a Tier III data center.

Tier IV facilities hardly ever experience downtime and when they do it is only for a fraction of a second while failovers switch to alternative systems. There are Tier IV certified data centers that count the days and hours since their last interruption with some spanning years.

As far as redundancy, a Tier IV facility typically features 2N+1 Redundancy. The term 2N+1 means that the facility has two times what is required to operate plus a backup.


Know the difference between the data center tier levels is important. However, is it worth the added cost difference? The answer depends on who you ask. It depends on how critical your applications and data are to your organization and stakeholders. An article published by Data Center Dynamics did just that. They asked the tough questions to a group of IT professionals and here are some important takeaways from what they said.

“It all depends on the kind of reliability and service level agreements with the end client under which the servers in the data center are hosted. If we are looking at robust and reliable data centers, Tier IV is a must, but if we go in terms of efficiency and infrastructure required, Tier III is more efficient and cost-effective.” Rahul Shewale, consultant, Capgemini, India [3]

“The expense, in all aspects required for a so-called Tier IV installation, is a waste in my opinion. You can build a Tier III(+) data center with a more reliable infrastructure than a “box standard” Tier IV facility. You need to consider the reliability and quality of the equipment, maintenance regimes, personnel training, staffing, procedures, safety, etc. Each of those can increase risk, reduce reliability, or increase it if done properly.” Dr Carlos Garcia de la Noceda [3]

“I have some depth of experience in the field, as I was part of the design team that built the first Tier IV facility in the world, and we knew at that time the reasons why we had decided to achieve a fault-tolerant, concurrently maintainable facility. The further development of the operational side of this field is as important as the design, given almost 60% of all infrastructure failures occur because of people, not because of infrastructure equipment failure.” William Angle, principal, CS Technology, Baltimore [3]

We would love to hear from you. What do you think about Tier III vs. Tier IV data centers? Is a Tier IV data center worth the extra expense? What do you take into consideration when selecting a data center? Is the tier level of the data center an important consideration?


[1] About Uptime Institute (About Uptime Institute) http://uptimeinstitute.com/about-us.

[2] Understanding Tier 3 and Tier 4 (Tier 3/Tier 4: datacentre classification)


[3] Do we really need TIER IV data centers? | Datacenter Dynamics (Do we really need TIER IV data centers? | Datacenter Dynamics)http://www.datacenterdynamics.com/focus/archive/2013/01/do-we-really-need-tier-iv-data-centers


Datacenters.com Technology

Datacenters.com is the fastest and easiest way for businesses to find and compare solutions from the world's leading providers of Cloud, Bare Metal, and Colocation. We offer customizable RFPs, instant multicloud and bare metal deployments, and free consultations from our team of technology experts. With over 10 years of experience in the industry, we are committed to helping businesses find the right provider for their unique needs. 


Subscribe to Our Newsletter to Receive All Posts in Your Inbox!