MTBF (Mean Time Between Failures) is a reliability measure: the average operating time between one breakdown and the next. Higher MTBF means a more reliable asset.
The MTBF formula
MTBF = Total Operating Time / Number of Failures
Step by step
- Choose a period and asset. Pick the asset and the time window you want to measure (for example one month of running).
- Add up operating time. Sum the time the asset was actually running during the period, excluding downtime.
- Count the failures. Count the number of failures (breakdowns) in the same period.
- Divide. MTBF = Total Operating Time / Number of Failures.
A worked example
A machine ran for 500 hours in a month and failed 5 times:
| Step | Calculation | Result |
|---|---|---|
| Operating time | - | 500 h |
| Failures | - | 5 |
| MTBF | 500 / 5 | 100 h |
Skip the spreadsheet
Open the MTBF / MTTR Calculator
Our free MTBF / MTTR Calculator does this live, with the benchmark overlay.
Common mistakes
- Including repair (downtime) hours in operating time.
- Mixing failures with planned stops or minor adjustments.
- Comparing MTBF across very different assets without context.
MTBF FAQ
Is a higher MTBF better?
Yes. A higher MTBF means longer average run time between failures, so better reliability.
What is the difference between MTBF and MTTR?
MTBF measures how often it fails (reliability). MTTR measures how long repairs take (maintainability).
How are MTBF and availability related?
Availability rises when MTBF is high (fewer failures) and MTTR is low (fast repairs).
Related: MTTR · preventive vs predictive maintenance · availability