# Mean time between failures

Free guide to maintenance metrics

## What is mean time between failures?

Mean time between failures (MTBF) is the average time between system breakdowns. Mean time between failures is a crucial maintenance metric to measure performance, safety, and equipment design, especially for critical or complex assets like generators or airplanes. It’s also used to determine the reliability of an asset.

MTBF is also one-half of the formula used to calculate availability, together with mean time to repair (MTTR). The MTBF formula uses only unplanned maintenance and doesn’t account for scheduled maintenance, like inspections, recalibrations, or preventive parts replacements.

## How do you calculate mean time between failures?

Calculating MTBF is pretty straightforward. You divide the total number of operational hours by the number of failures in that period. MTBF is usually measured in hours.

### MTBF calculation

MTBF = # of operational hours ÷ # of failures

For example, an asset may have been operational for 1,000 hours in a year. For that year, that asset broke down eight times. Therefore, the MTBF for that piece of equipment is 125 hours.

To get an accurate measure of MTBF, you must collect data from the actual performance of the equipment. Each asset operates under different circumstances and is influenced by human factors, such as design, assembly, maintenance, and more. That’s why you should avoid basing your maintenance around an MTBF estimate from a manual.

#### Mean Time Between Failure Calculator

= (# of operational hours / # of failures)

## Why do we calculate MTBF?

MTBF is used to anticipate how likely an asset is to fail within a specific period or how often a particular type of failure may occur. When paired with other maintenance strategies like failure codes, root cause analysis, and additional maintenance metrics like MTTR, it will help you avoid costly breakdowns. Calculating MTBF makes it easier to create preventive maintenance strategies, so reliability can be improved by tackling issues before they cause failure. If a failure does occur, having all the data allows you to improve maintainability.

## What does MTBF mean for maintenance?

Failure is a problem, and knowing everything about it is often the best way to solve it. Measuring and calculating MTBF is one way to get more information about a failure and mitigate its impact. Conducting an MTBF analysis helps your maintenance team reduce downtime while saving money and working faster.

Calculating an asset’s MTBF provides a baseline for maximizing your preventive maintenance schedule. Knowing approximately how often an asset fails allows you to schedule preventive maintenance before that point. This gives you a better chance to prevent failure while doing as little maintenance as possible and maximizing your resources. This method is a good step towards condition-based maintenance.

You can also go a step further and measure MTBF for a specific failure. Not only can you target that failure with preventive maintenance schedules, but you can also investigate why a specific issue results in a lower MTBF. The culprit can be anything from vague task lists to defective parts or inadequate training. With this knowledge, you can pinpoint and eliminate the root cause of a particularly consistent failure.

PMs aren’t the only task that can be optimized using MTBF. Inventory management can also be improved by tracking this maintenance metric. Knowing approximately how long you have before a piece of equipment goes down can fine-tune your approach to MRO inventory purchasing. For example, you can get a better sense of minimum quantities and lead times to achieve just-in-time delivery, resulting in lower costs and quicker repair times.

Lastly, MTBF can help make tough decisions a little easier. It’s never easy to give up on a piece of equipment and buy an expensive new machine. However, if all attempts to combat low MTBF are unsuccessful, it might be in your best interest to replace the asset instead of spending time and money repairing it all the time. MTBF can be used to calculate the cost of repair vs. replacement and make a business case for new equipment.

## What are some advantages of measuring MTBF?

There are many benefits in measuring mean time between failures, including:

• It helps manufacturers to determine the expected life of the product and also the reliability of the product.
• It identifies potential risks to their operations and plan for maintenance and repairs.
• It indicates the quality of parts and materials used in the production process.
• It can be used as an indicator of the effectiveness of preventive maintenance and repair processes.
• It can compare the performance of different models and brands of the same product.

## What are some of the disadvantages of measuring MTBF?

Mean time between failures does not tell the whole story; it does not provide information about the causes of the failure or the severity of the failure. It can also be skewed by outliers, as a single event can drastically change the mean value.