MTBF, MTTF, MTTR Calculation: Failure Metric Guide & Formulas

The Role of MTBF, MTTF, and MTTR in Maintenance Performance Metrics

Mean time between failure (MTBF), mean time to failure (MTTF), and mean time to repair (MTTR) are three commonly used maintenance key performance indicators (KPIs). Each of these particular maintenance KPIs acts as a lagging indicator that provides insight into past events. Even though these indicators display information about what happened in the past, they provide key information organizations can use for planning inventory needs, maintenance strategy, scheduling, budgeting for asset replacement, and other decisions. 

Maintenance KPIs help organizations track performance related to strategic goals and objectives. This information lets maintenance professionals unravel process problems, facilitate data-driven decision-making, and make adjustments specific to the findings. In this article, we’ll show how to calculate MTBF, MTTF, and MTTR and illustrate how each KPI can support strategic maintenance planning. 

What is MTBF / MTTF Calculation? Failure Metrics for Maintenance Reporting

MTBF and MTTF are both failure metrics. They are performance indicators that measure the reliability and availability of assets or individual components. The MTBF MTTF calculation uses the same information, but they have different implications depending on whether the asset is repairable or not.

MTBF MTTF calculation = Total uptime ÷ number of failures or Total operating hours ÷ number of assets 

MTBF (mean time between failures) and MTTF (mean time to failure) calculations are essential metrics for system reliability that each have their own separate formulas. To find MTBF, divide the total operational time by the number of failures. This measurement provides insight into the average time between system failures. Conversely, MTTF measures the average operational lifespan of non-repairable systems by dividing the total operational time by the number of devices, offering a snapshot of expected device longevity.

Mean time between failures calculates the average length of time an asset or component runs before it needs repairs. This KPI is used for repairable assets.

Mean time to failure is used to calculate the time a part or asset lasts before it needs to be completely replaced. It’s used for assets that can’t be repaired, or for which repair would be less cost-effective than replacement. This KPI applies to parts like lightbulbs, seals, gaskets, fuses, and some types of filters.

When doing either of these calculations, it’s important to include only assets that are exactly the same. For example, in a MTTF calculation for gaskets, you’d want to use data about gaskets made of the same material and used in the same asset to ensure your MTTF calculation is accurate. The same goes for calculating MTBF — you would want to only use data for the exact same type of asset or individual component.

Here’s a more detailed breakdown of MTBF, MTTF, and MTTR, and why each of these KPIs are essential information for making strategic maintenance decisions.

What is MTBF?

All machines will eventually fail. Mean time between failures depicts the expected time between two failures for a repairable system. Simply put, MTBF predicts how long a piece of equipment can operate without disruption. MTBF is one of the most efficient maintenance KPIs for analyzing asset reliability and forecasting future performance. Higher MTBF means a more reliable system.

Benefits of Tracking MTBF

Tracking MTBF allows teams to optimize their preventive maintenance (PM) scheduling. A shorter than expected MTBF could indicate that preventive maintenance needs to be done more frequently or that condition-based monitoring should be added in order to extend the MTBF.

It also allows teams to predict how often failures will occur during production. This can help teams set realistic production goals by enabling estimates for when an asset might need repair. And by tracking when assets are expected to need maintenance, maintenance managers can schedule maintenance tasks more effectively.

By using MTBF along with inventory management software, maintenance teams can improve their spare parts inventory and availability. MTBF reports allow teams to estimate when replacement parts will be needed and can help ensure the correct replacements are on hand, reducing downtime.

How Do You Calculate MTBF?

You can calculate MTBF by taking the total time a machine is running (uptime) and dividing it by the number of breakdowns during that same period (failures).

MTBF calculation = Total uptime ÷ number of failures

While some manufacturers may offer their own version of the expected equipment lifespan, it’s important to calculate this number yourself based on data from your internal systems. MTBF can be highly variable based on the asset’s operating conditions and other factors.

MTBF vs. MTTF and MTTR 

MTBF is different from MTTR and MTTF in a few key ways. While all three metrics are important maintenance KPIs, mean time between failures (MTBF) focuses on the intervals between failures, or the amount of time you can reasonably expect an asset to run without needing repairs. This is an important measurement for the reliability of a system. 

MTTR, or mean time to repair, focuses on the amount of time it takes to repair a system. It is almost the opposite of MTBF since the MTTR calculation uses the time the asset is not running, while the MTBF calculation uses the time the asset is running.

MTBF and MTTF, or mean time to failure, are very similar. But MTBF is used for assets that can be repaired, while MTTF is used for assets or components that must be replaced.

What is MTTR?

Mean time to repair (MTTR) measures the maintainability of repairable machines and components. It calculates the average time to fix a failed asset, including the time it takes to test and diagnose. MTTR can help reliability leaders see how much time technicians take to repair a specific machine and examine why a particular repair takes more time than expected. The goal is to keep MTTR as short as possible to reduce equipment downtime.

Maintenance operations can reduce MTTR by analyzing their equipment metrics and repair processes to determine where and how to lower downtime and optimize the process by standardizing preventive maintenance tasks.

Benefits of Tracking MTTR

Tracking MTTR leads to many organizational benefits.

First, it helps teams understand their capacity to react to failures. A long MTTR can indicate several different problems, and knowing the MTTR makes it easier to identify problems and determine how to correct them. 

For example, a higher MTTR number could indicate a resource problem, such as not having enough maintenance technicians or lacking the proper tools or replacement parts in inventory. It can also identify process bottlenecks, such as work order management problems or communication delays between team members. When organizations recognize these problems, they can take steps to correct them, resulting in a reduction of MTTR.

Tracking MTTR also allows for improvements in maintenance planning. By identifying how long repairs usually take, maintenance teams can better understand how and where to schedule their resources. High MTTR values can indicate the need for improved preventive maintenance practices. As a result, these strategies can reduce repair frequency and unplanned downtime.

Organizations can use MTTR to make data-driven decisions about how and where to invest in their maintenance program. It provides a quantifiable metric to justify investments in hiring additional staff members, providing education or cross-training for the current team, or implementing programs like predictive maintenance. 

Calculating MTTR is the first step to improving the maintenance processes. Ultimately, it can have a major impact on production, downtime, and operational costs.

Formula for MTTR

Calculate mean time to repair by adding up the total amount of time the asset was offline for unplanned maintenance from the time the asset went offline to the time it was completely restored. Then divide that number by the total number of repairs. 

MTTR calculation = Total maintenance downtime ÷ number of repairs

Remember that the total maintenance downtime may include time spent troubleshooting, testing, and waiting for replacement parts. 

MTTR vs. MTBF and MTTF

MTTR is different from MTBF and MTTF because of its focus. Instead of being a reliability metric concerned with the length of time an asset or component should last, it focuses on the length of time it takes to get the asset or component back up and running.

The length of time it takes to repair an asset doesn’t include just the hours a maintenance technician spent performing the repair. Instead, it may also include downtime spent waiting for a replacement part to arrive or time spent waiting for a technician with the correct skill set to become available to work on the problem. The MTTR calculation may have multiple factors in play, which makes it one of the more critical KPIs to analyze and understand in order to improve overall plant maintenance. 

What is Mean Time to Failure (MTTF)?

Mean time to failure (MTTF) measures equipment reliability and the time between one failure and the next. It is the mean time anticipated until the machinery or component fails and needs to be replaced. MTTF applies only to non-repairable assets, i.e., machinery that either cannot or should not be repaired, such as a light bulb.

Lengthening MTTF times can significantly impact your bottom line by minimizing operational disruptions, maximizing asset life cycles, and enabling more effective operations and maintenance decision-making. Likewise, shorter MTTF times can cause missed orders, unfulfilled business objectives, or harm to your organization’s reputation.

Benefits of Tracking MTTF

There are many benefits of tracking MTTF, particularly for companies that focus on preventive maintenance and reliability. 

Tracking MTTF allows organizations to plan for replacement of consumable parts like bearings and filters before failure occurs. This can help minimize downtime, since these parts can be replaced during planned downtime when they near the end of their expected lifespan. Replacing components before they fail also enhances safety by reducing the likelihood of catastrophic failure.

MTTF data helps forecast the replacement rate of non-repairable components for budgeting purposes. Knowing MTTF assists with inventory planning, ensuring the correct replacement parts are on hand without unnecessarily overstocking.

If the MTTF of a specific part or component suddenly decreases, it could indicate a reduced quality in replacement parts. If this occurs, organizations can look for a different vendor with higher quality products in order to reduce unplanned downtime and remove unnecessary replacement costs.

How do you Calculate Mean Time to Failure (MTTF)?

Calculate mean time to failure by dividing the total hours of operations by the total number of machines in use.

MTTF calculation = Total number of operating hours ÷ number of assets

When calculating MTTF, it’s important to use data from components that are exactly the same so you can be sure your calculation is accurate. For example, when calculating the MTTF for light bulbs, use only lightbulbs with the same wattage.

MTTF vs. MTBF and MTTR 

The relationship between MTTF (mean time to failure) and MTBF (mean time between failures) centers on their application to different types of systems. MTTF is used for non-repairable systems, indicating the average time until a system fails permanently and requires replacement. Conversely, MTBF is used for systems that are expected to be repaired and returned to service, reflecting the average time between failures. In scenarios where failures always result in replacements, MTTF can be analogous to MTBF, highlighting their conditional interchangeability.

Calculating MTTF from MTBF is not directly feasible as these metrics apply to different types of systems — non-repairable and repairable, respectively. However, in theoretical terms, MTTF can be considered equivalent to MTBF for components that are not repaired. To use MTBF data for MTTF instead, assume that all system failures lead to replacements rather than repairs.

Calculating and Reporting on Failure Metrics With a CMMS

Calculating KPIs like MTBF, MTTR, and MTTF requires an enormous amount of accurate, accessible data. With a computerized maintenance management system (CMMS), this data can be turned into actionable reports with just a few clicks. A CMMS simplifies the reporting of failure metrics like MTBF, MTTR, and MTTF and can also track other maintenance KPIs and enable automated progress reports using stored data from sources like work orders, inventory management software, and integrations with other systems like PLC and BMS systems. 

These results help reliability professionals optimize maintenance programs and processes and implement predictive strategies like condition-based maintenance (CBM). CBM lets maintenance teams more effectively pinpoint when an asset needs attention and, when paired with KPIs, gives an extra boost to achieving an organization’s objectives.

CMMS software can be configured to match how organizations track their unique MTTR, MTBF, and MTTR data. And with a CMMS, hundreds of other KPIs beyond these metrics are available. Organizations can determine the most effective KPIs for their needs and use reporting and analytics dashboards to build customized reports to track vital maintenance KPIs. 

A CMMS breaks down the data silos that traditionally exist between teams and gives you the information you need to refine maintenance processes, reduce downtime, and cut costs by improving your organization’s KPI metrics.

But measuring the metrics alone doesn’t improve processes. When working to improve any KPI, organizations should ask these questions: 

• What are the objectives?
• How will they be achieved?
• Who will act on the data?

Answering these questions and designing an action plan based on the insight gathered from a CMMS guides organizations towards actionable steps that result in improved KPIs, reduced downtimes, and more efficient operations. 

To see what a CMMS can do for your organization, try a free trial of eMaint CMMS today.