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Mean time to failure

(also MTTF)

Mean time to failure definition

A mean time to failure (also known as MTTF) is a metric that estimates the average time a device, system, or product is expected to last before its first failure. MTTF is used for products that users typically need to replace after their first failure rather than fixed (and is considered a product or system’s average lifespan). MTTF is calculated by analyzing historical failure data or conducting reliability tests.

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How is MTTF calculated?

  • The first step is gathering data about the system’s failures over a certain time period. Typically, you can get this data from testing, observations, historical records, or simulations.
  • Then, add all the times between each failure (or the time intervals).
  • Next, count the number of failures during the observation period.
  • Finally, calculate MTTF by dividing the total cumulative operational time by the number of failures.


  • MTTF (mean time to failure) and MTBF (mean time between failures) are reliability metrics that evaluate how well products perform and last.
  • However, MTTF measures the average time a device or system is expected to work before failing for the first time, while MTBF calculates the average time between consecutive failures.
  • MTTF is mostly used for analyzing products that are impossible to fix and need to be replaced after their first failure. The MTBF metric, on the other hand, is used for repairable products.