How to Calculate the MTBF

By Andrea Peck; Updated April 25, 2017

Reliability engineering began in the aviation industry, where unreliable equipment often meant the difference between life and death. MTBF, or mean time between failure, is a statistical model that has become a common tool used to quantify reliability in the assessment of mechanical products. Increasingly, MTBF is employed in mechanical and system design, product procurement, plant operations and product and plant maintenance.

MTBF Basics

The MTBF is a mathematical model that describes the mean or average time that a machine operates before repairable failure occurs. Inherent in the definition of MTBF is the idea that repair will occur. MTBF is perhaps the simplest and most accurate method of determining the reliability of an engine or other type of machinery. Reliability is the likelihood that a device will function optimally for a specific length of time. The integrity and quality of a product are an integral part of MTBF.

Calculating MTBF

To determine the MTBF, a trial period where machinery runs to its designed purpose and then fails is required for accurate assessment. MTBF = t/r where t = total run time and r = number of failures during that time. Total run time (t) is the sum of operational times. For example, if a system runs for 50 hours before failure, then 48 hours and then 51 hours, the total run time would be 50 + 48 + 51 = 149. Failures (r) would be 3 in this example, which would give an MTBF of 149/3 or approximately 49.7 hours of operation before failure occurred. It is important to note that when collecting data, mean operation time includes only the time when machinery is functioning. It does not include time for maintenance, repair or or otherwise nonoperational hours.

Influencing Factors

MTBF is considered an important marker of reliability, but it does have limitations. MTBF predictions are commonly practiced during the normal life period of a device. Normal life period is defined as the time when the lowest rate of failure occurs. As machinery ages, failure rate naturally increases. Because of this, most MTBF data is not averaged in. Additionally, reliability tests are performed under advantageous circumstances where dust, humidity, temperature, movement and other factors are controlled. Though reliability engineers consider the normal user and everyday factors of use, discrepancies such as shortened MTBF sometimes occur in real-world scenarios.

Formula in Action

Whether you are interested in purchasing a new computer, a new car or an entire factory full of equipment, the MTBF of a product is a tool that can inform the decision-making process. For those who work with mechanics, knowledge of MTBF provides insight into challenging repairs. Knowing the MTBF of parts and components allows maintenance schedules to be created before problems occur.