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Mean Time To Failure (MTTF): what is it used for and how is it calculated

The MTTF (Mean Time To Failure) parameter measures the average operating time of a non-repairable asset before it breaks. Find out what it is and how it differs from other important indicators such as MTBF and MTTR

Anticipating the occurrence of a fault that could cause the malfunction or even put an organization’s plant and equipment out of use is one of the most important but, at the same time, complicated activity for a facility manager.

Fortunately, there are valuation metrics, including MTTF, MTBF, and MTTR, that are specifically designed to measure assets reliability and lifespan, which can help facility managers make decisions more consciously and minimize business disruptions.

In this article, we will explore the meaning of Mean Time To Failure (MTTF) and try to understand what this parameter is used for, providing the necessary information to learn how to calculate it correctly.

To obtain reliable data on which to set the MTTF calculation, or to automatically track the performance indices adopted in maintenance, I recommend using a facility management software, a centralized platform in the cloud, accessible anywhere and from any device, which allows you to monitor the operation of the assets and helps you plan preventive actions aimed at prolonging the life cycle of the individual equipment.

What is Mean Time To Failure (MTTF)

Mean Time To Failure (MTTF) is a statistical parameter used in the Facility Management sector to assess the reliability of mechanical and electronic systems. This parameter helps to quantify the average operating time of non-repairable equipment before it breaks and needs to be replaced. Considering the aspect that the MTTF parameter only applies to assets that cannot be repaired, it can also be considered as the average useful life of the asset.

The reasons why an asset (or a part of it) may not be repairable are different, but the most common one lies in the too high times and costs of some types of repair, which make it more convenient to replace the damaged component rather than repair it. In many cases, however, there may also be technical limitations to carry out repairs (as is the case, for example, for a worn-out bulb or a broken transmission belt, in these cases the entire component needs replacing).

As you may have noticed, the MTTF index usually refers to the smallest, replaceable component of a plant system, which are intended to keep larger, repairable resources running. Knowing and understanding the Mean Time To Failure is of fundamental importance for organizations and facility managers who intend to improve their maintenance strategies, adopting a proactive attitude that helps them prevent any problems, reducing unscheduled downtime and optimizing asset performance and duration.

MTTF schema Mean-Time-To-Failure

MTTF Mean Time To Failure

How to calculate the MTTF parameter

The Mean Time To Failure is determined by monitoring a large number of items of the same type for an extended period of time, with the aim of tracking their performance, operation and duration.

Since the MTTF parameter is used to express the average life cycle of these products, its value can be calculated by dividing the total sum of operating time for each individual asset by the total number of assets being monitored, as provided for by the following formula:


Total operating time

Total number of assets in use

The MTTF value is usually expressed in hours, so the larger its value, the longer the average life span of the asset will be.

MTTF Calculation Example

Suppose you want to determine the Mean Time To Failure for a specific type of lamp used in a lighting system.

The lamps considered in the MTTF calculation have the same characteristics (manufacturer, size, power, etc.) and are a total of 7, each of which has reached a lifetime in hours of 25, 20, 19, 22, 24, 16, 21, respectively.

By adding the duration of the individual lamps, a total operating time of 147 hours is obtained, which we can insert in the formula illustrated above:


Total operating time

Total number of assets in use




= 21 hours

This means that the lamps are capable of operating on average for 21 hours before they reach the end of their useful life requiring replacement.

How to use the MTTF parameter

The MTTF value helps professionals, engaged in asset maintenance, to predict how long their assets will be able to function properly, before undergoing a permanent shutdown. Calculating and monitoring this parameter can therefore be extremely useful in the following cases:

  • for scheduling inspections and preventive maintenance activities, with the aim of anticipating any problems, minimising the occurrence of faults, breakages or sudden malfunctions, and maximising the useful lifetime of resources;
  • decision making aspects when purchasing components and equipment, as items with a higher MTTF ensure higher quality and durability;
  • developing just-in-time inventory (JIT) strategies, a management system that allows companies to receive goods and spare parts when they are actually needed. This system reduces the need to store large quantities of stock, resulting in improved efficiency and cost savings.
MTTF: main functions

MTTF: main functions

MTTF, MTBF and MTTR: what are the differences

MTTF, MTBF and MTTR are three different maintenance metrics used to assess the reliability and service life of certain systems or components. In particular the below:

  • the Mean Time To Failure (MTTF) specifies the average operating time of equipment before suffering a failure and is used only for those assets that cannot be repaired. This is basically why it coincides with the life cycle of the asset itself;
  • Mean Time Between Failure (MTBF) measures the average operating time of the equipment between multiple failures or interruptions and, unlike MTTF, is applied to repairable resources;
  • Mean Time To Repair (MTTR) refers to the amount of time required to repair a system and restore its full functionality.

How to track maintenance metrics

The most effective method to track these parameters is to use specific Facility Management software that helps professionals in the sector to cover the following tasks:

  • collect up-to-date and accurate information from each resource;
  • keep the performance indicators of the equipment under control;
  • reduce the downtime of the various systems;
  • plan interventions and maintenance operations;
  • create customized reports that help you improve your decision-making processes.

If you also want to optimize your work and implement maintenance monitoring processes, I recommend you try the demo version of a Facility Management software and discover how easy it can become to manage maintenance processes.