MTBF from Runtime + Failure Count
Turn logged operating hours and failure counts into MTBF, failure rate, and availability.
Your recent runs (stored only in your browser)
No calculations yet — results land here so you can compare runs.
The engineering
MTBF is just accumulated operating time divided by the number of failures observed over that time — it is a rate estimate, not a guaranteed lifetime. Reach for it when you have a fleet log: sum every unit's runtime, count the failures, and divide. Multiply units × hours so a 5-unit line running 2,000 h is credited with 10,000 unit-hours.
The classic trap is reading MTBF as "how long one unit lasts." A 100,000-hour MTBF on a part with a 5-year wear-out life does not mean 11 years of service — it means that during its useful (constant-rate) life the instantaneous failure rate is 1/100,000 per hour. MTBF only applies to the flat middle of the bathtub curve. With zero failures logged you cannot divide, so fall back to a χ² confidence bound instead.
Where this math comes from
MTBF grew out of WWII electronics maintenance, where vacuum-tube gear failed so often that the Navy and Air Force needed a common number to compare designs and budget spare parts. Post-war, the 1957 AGREE report (Advisory Group on Reliability of Electronic Equipment) formalized reliability as a specifiable, testable requirement rather than an afterthought.
The Department of Defense codified the exponential-life prediction methods in MIL-HDBK-217 for part-level rates and MIL-HDBK-338 for the system-level reliability engineering that ties runtime and failure counts to MTBF and availability. Those handbooks are still the reference every industrial reliability engineer quietly cites.
- 1957AGREE (DoD)Reliability report establishes MTBF as a specifiable, demonstrable requirement.
- 1962US DoDMIL-HDBK-217 issued — standardized component failure-rate prediction.
- 1988US DoDMIL-HDBK-338 consolidates electronic reliability design, including MTBF and availability methods.
- 1998US DoDMIL-HDBK-338B revision — the version cited on most current programs.
See the full timeline of the math behind every calculator →
Runs entirely in your browser — nothing you enter leaves this page. Your recent runs are stored only on your device.