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System Availability: what is it and the difference from reliability metrics

The availability of a system is a metric that serves to assess the likelihood that an asset will be available for use when needed. Find out how to calculate it

The efficiency of a production system depends largely on the amount of time in which the machines are able to function properly and smoothly.

To evaluate the performance of their plants, companies and Facility Management professionals can count on the help of some important performance indicators. Among these, the so-called availability metrics that have the objective of determining the level of operation of the assets.

Before showing you how to calculate these maintenance metrics, I recommend that you immediately rely on the potential of a Facility Management software to monitor the operating conditions of your equipment. Using this system will help you get up-to-date information in real time to assess the availability of your resources. This also provides you with the support you’ll need to plan, monitor and manage any maintenance activities.

What is System Availability?

System availability is a metric used in maintenance to measure the amount of time, expressed as a percentage, that an asset can be used for production.

The calculation of this performance index is fundamental for those companies that focus their activity on plants systems and machinery. The availability analysis allows to quantify the probability that a system is not faulty or no need repairs when it is necessary to use it.

To be “available” an asset must meet the following three conditions:

  • ensure proper functionality, therefore, avoiding out of service conditions for inspection or repair work;
  • ensure normal operation by operating under provided conditions (for example, at a given speed) to achieve the intended purpose;
  • be readily usable when required, therefore not interrupting production schedules.
MMonitoring maintenance metrics - Difference between availability and reliability

Monitoring maintenance metrics

Why Availability Monitoring is important

Measuring availability allows to determine whether the systems can deliver the services they expect and production potential is maximized.

Thoroughly evaluating this metric and knowing how to respond to it appropriately can have a significant impact on the organization’s economy. When availability is high, it means that the equipment can remain operational for as long as possible at maximum performance. This result in an overall increase in efficiency, productivity and final revenues.

The availability of resources can be influenced by several factors. These include reliability, which we will see later in the article, and maintainability, which measures the ability to maintain or restore an equipment to its normal operating state in the shortest possible time. Since these two factors depend on the maintenance strategies adopted, the evaluation of availability also becomes essential for:

  • study and measure the effectiveness of existing maintenance practices and programs;
  • proactively detect problems;
  • identify opportunities for better maintenance planning (whether preventive or corrective);
  • provide the necessary information and tools to deal with incidents in an appropriate manner.

What is the difference between availability and reliability?

The concepts of availability and reliability are often used interchangeably but they really have completely different meanings.

Availability metrics serve, to quantify the time in which a system is actually available for use. In this sense it needs to consider the intervals in which the asset is inactive because it is faulty, or subjected to repairs or simple routine maintenance.

Availability can be considered a parameter that takes into account the totality of the downtime of a system. It includes both planned and unplanned downtime. It therefore takes in account the time necessary to perform scheduled maintenance and related inspections, and the downtime due to the occurrence of failures, sudden interruptions and unexpected malfunctions.

Reliability of an asset, unlike availability, represents the absence of unplanned downtime. It refers to the likelihood that a system can operate without failure under normal operating conditions for a predetermined period of time. An equipment can therefore be “reliable” if it works as intended each time it is to be used.

To better understand the difference between availability and reliability we can refer to the following example: an asset that never fails and is not subjected to unplanned downtime in the observation period, can be considered 100% reliable. The same asset could, however, provide an availability of 90% if every hour out of ten is inactive due to routine inspections and scheduled maintenance interventions.

Difference between availability and reliability

Difference between availability and reliability

How to calculate System Availability ?

In the availability analysis, two essential components come into play:

  • the asset’s up-time, i.e. the total time in which the equipment is actually operational and operational;
  • the sum of system up-time and downtime, calculated on planned and unplanned downtime.

To assess the availability, simply divide the two components according to the formula below, expressing the result as a percentage:

Availability of the system =


Up-time + Down-time


Suppose, for example, you want to calculate the availability of a conveyor belt that has worked properly for 120 hours in a week. We assume that during this period the asset has suffered two failures that have required respectively 10 and 20 hours for their repair (equal to 30 hours of total inactivity). Applying the formula seen above, the availability of the conveyor belt will be equal to:

Availability =


120 + 30

x 100=80%

Alternative method for calculating availability

An alternative procedure for calculating availability is to use maintenance metrics that measure failure rates and reveal the effects of malfunctions on equipment up-time:

  • the mean Time Between Failure (MTBF), which describes the average operating time of an asset between two repairable failures;
  • the mean Time to Repair (MTTR), which refers to the amount of time it takes to repair an asset.

Once these indicators have been evaluated, to determine the availability of the equipment in question, apply the following formula:

Availability of the system =



How to improve System Availability

When an equipment is not available at the right time in a company, is easy to run into problems. This issue certainly can cause delays, interruptions, waste and inconveniences of various kinds. Ensuring a good level of availability is, therefore, an essential condition to ensure the success of an organization.

In order for the level of availability of the equipment to be appropriate (on average above 90%), it is important to adopt proper strategies, which include:

  1. the development of design processes base on the prediction of failure, which allow the elimination of potential errors already during the creation of assets;
  2. risk mitigation, through the implementation of a criticality analysis that aims to identify the consequences of malfunctioning systems well in advance;
  3. the constant monitoring of the parameters relating to failure rates and operating conditions and use of the equipment;
  4. the optimization of preventive maintenance programs, to reduce the chances of error and avoid unplanned downtime;
  5. the creation of standard protocols aimed at solving the problems encountered with greater efficiency and effectiveness.

To implement these strategies in the simplest and most effective way, let yourself be supported by a Facility Management software, a centralized and cloud-based system that will allow you to keep track of every aspect related to maintenance. This it also provide you with always up-to-date and reliable information. A Facility Management software will help you develop the necessary actions to increase the level of availability of your equipment.