What does MTBF stand for?
MTBF stands for Mean Time Between Failures. The average operating time between failures of a repairable component. A higher MTBF means more reliable hardware.
MTBF is the average time a repairable system runs between one failure and the next. It measures reliability: the higher the number, the less often the system fails. You calculate it by dividing total operating time by the number of failures. It says nothing about how long each repair takes; that is MTTR.
MTBF in practice: an example
A storage array ran for 6,000 hours in a year and failed three times. Its MTBF is 6,000 divided by 3, which is 2,000 hours. If each repair took 4 hours on average, MTTR is 4 hours, and availability is MTBF divided by MTBF plus MTTR: 2,000 divided by 2,004, about 99.8%.
What MTBF is confused with
MTBF vs MTTR: what is the difference?
MTBF is how long it works between failures; MTTR is how long it takes to fix. You want MTBF high and MTTR low. What MTTR stands for
MTBF vs MTTF: what is the difference?
Mean time to failure is used for items that are replaced, not repaired, such as a disk drive. MTBF is for systems that are repaired and put back into service.
Which exams cover MTBF?
MTBF is on the official CompTIA acronym list for two of the four exams covered here.
- Security+ SY0-801: 3.4 (Security Architecture).
- Network+ N10-009: 3.0 (Network Operations). Listed there as “Mean Time Between Failure”.
Each number is a CompTIA exam objective and the name in brackets is its domain. The exam puts the term in a scenario; it does not ask you to recite the expansion.
Check yourself: two practice questions
Written by CertSprint for this page. They are not taken from any exam.
1. A router operated for 9,000 hours and failed three times. What is its MTBF?
- 300 hours
- 3,000 hours
- 9,000 hours
- 27,000 hours
Show the answer
B. 3,000 hours. MTBF is operating time divided by number of failures: 9,000 / 3 = 3,000 hours.
2. Two firewalls are compared. Which figure shows that model A fails less often than model B?
- A higher MTBF
- A higher MTTR
- A lower RPO
- A shorter warranty
Show the answer
A. A higher MTBF. MTBF measures time between failures, so a higher value means fewer failures. MTTR is about repair time.
Sources: NIST glossary: mean time to failure (the related measure for items that are replaced); NIST SP 800-34 Rev. 1, Contingency Planning Guide. The acronym and its expansion come from CompTIA's official acronym lists, linked from each exam glossary above. The explanation, example and questions are CertSprint's own, checked against these sources on 11 November 2026.