How to Calculate I.T. Service Availability

I know…I just wrote about I.T. availability so why am I writing yet another post on the subject? My recent post on this topic entitled  I.T. AVAILABILITY MANAGEMENT (FACTS & FICTION) shared some high level perspectives about I.T. Availability for business managers and I.T. professionals alike.

This post focuses a little more on equipping I.T. managers. It digs a little more deeply into the topic of I.T. Availability Management in that it offers brief tutorial on how to calculate the availability of an I.T. service. After the tutorial I also provide some practical application for the formula in order to help you gain value from the information.

Those of you who are familiar with I.T. Management standards know that measuring I.T. service availability alone doesn’t necessary offer a complete measure of I.T. service reliability and maintainability. See How to Calculate Reliability and Maintainability to calculate I.T. service reliability and maintainability of an I.T. Service.

Availability Definition (ITILv3)

Ability of a Configuration Item (CI) or IT Service to perform its agreed Function when required.

How to Calculate I.T. Service Availability (ITILv3)

To measure the availability of a service, you can subtract the amount of downtime from the Agreed Service Time (AST), then divide the result by the AST. You can then multiply the number by 100 to obtain the percentage.


How To Calculate Acceptable Downtime From a Known Availability Percentage

To calculate the accepted downtime from a known availability percentage, Multiply the numeric value of the percentage by the AST and divide it by 100. You can then subtract the result from the AST.


A quick note about Agreed Service Time (AST)

Many people defer to 24 hours/7 days a week as an agreed service time by default. The reality is that many I.T. services don’t actually need to be available 24 hours a day 7 days a week. As a business or I.T. manager, it is important to take a candid look at different I.T. services that you rely on to operate your organization. For example, email in your organization will probably need to be available 24 hours a day/7 days a week. However, payroll may only need to be available 10 hours a day 5 days a week. I.T. may offer access to the payroll system 24/7 as a convenience to staff. However, it may cost more to warrantee such availability than it is worth. With this in mind, it is important to have and communicate Agreed Service Times that outline when users/customers can expect a service to be “fit for use” and available to perform their functions as expected. Availability calculations should only be applies using the total amount contained within the Agreed Service Time.

Why calculating and measuring I.T. availability important?

With all of these details, you might be asking why this is so important? After-all in the cloud world nothing ever really goes down anyway right? Wrong. It may be true that we live in a world that ranges from 24/7 pharmacies to 24/7 fast food restaurants. However, technology still burns resources and costs money to operate. Technology still requires good leadership and even in this time of the perceived continuous availability that we observe in our cloud service world, technology fails.  With this in mind here are a couple practical applications for measuring I.T. availability.

  1. As an I.T. manager/professional you know that there can be a great deal of drama surrounding the availability of key I.T. services. You know that providing facts and data is a great way to mitigate drama and communicate growth. Therefore, as part of a monthly performance report to senior management you provide an availability report that highlights the availability of key I.T. services. This report may show that availability has improved as a result of a capital investment in technology resources.  Or, perhaps you want to communicate on a Quarterly basis that your technology infrastructure/staff is maintaining availability as a key contributor to the productivity and continuity of your organizational initiatives. In any case an I.T. manager should never take for granted or assume that senior management is clear about the availability of a service. They may hear complaints about a service failure. But they rarely hear about positive I.T. accomplishments. It is your job as the I.T. leader to communicate in measurable and objectives ways that the technology you steward is a good investment of organizational resources.    (Please Note: Some of you might want to use availability statistics to help you acquire funding in order to make I.T. improvements. Stay Tuned…I will be talked about using stats to acquire funding in future posts when write about write about Reliability. and Maintainability.)
  2. As a business or I.T. manager you are looking at the Service Level Agreement from a Software as a Service company that specializes in Sales support. They claim that they agree to warranty the availability of their service 99.9% of the time to be measured over a thirty day billing cycle. Having an understanding of how to calculate availability allows you to confirm how much time the service may be unavailable over the billing period.


Candidly computer users and customers that you support don’t care about best practices in I.T. They really only care about the value that you add to their lives through technology. For example, few people really care about what techniques a professional guitar player uses in a performance. They really only care about how good their “playing” sounds. I mention this in conclusion of this post in order to acknowledge that excellent I.T. management is about adding value and that sometimes I.T. managers get too caught up in process and lose sight of value. Furthermore, I.T. managers can get too caught up on using Availability management as a way to create barriers for users. It doesn’t have to be this way. As I.T. managers we must constantly ask the question, “How can we add value with technology?” Good management, order, discipline and good stewardship all add great value if managers keep this question in mind when they plan their I.T. availability management strategies.

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