How it is calculated
MTBF = Operating time ÷ Number of failures
MTTR = Total repair time ÷ Number of failures
Inherent availability = MTBF ÷ (MTBF + MTTR)
Failure rate λ = 1 ÷ MTBF
Reliability R(t) = e^(−t ÷ MTBF)
Worked example
A pump that ran 1,000 hours with 4 failures and 20 hours of total repair has an MTBF of 250 h, an MTTR of 5 h and an inherent availability of 98.0%. The probability it runs 100 hours without failing is about 67%.
Practical tips
- Count only operating time for MTBF, not calendar time, or you will overstate reliability.
- Raise MTBF with preventive maintenance and root-cause analysis; lower MTTR with spare parts on hand and clear repair procedures.
- The reliability formula assumes a constant failure rate, which fits most electronic and many mechanical assets in their useful-life phase.
Frequently asked questions
What is a good MTBF?
It depends entirely on the asset and industry. Compare the same asset type over time, or against the manufacturer's data, rather than against a universal number.
What is the difference between MTBF and MTTF?
MTBF is used for repairable assets that return to service. MTTF (Mean Time To Failure) is used for non-repairable items that are replaced, like bulbs or bearings.
Does MTTR include waiting for parts?
Definitions vary. Strictly, MTTR is active repair time. Many plants track the full time from failure to restart, which also includes waiting for the technician and parts. Use one definition consistently.