MTBF Calculator (MTTR, Failure Rate, Availability)
Calculate MTBF from field data or use a rated MTBF to get failure rate, FIT, MTTR, availability, reliability over a mission time and annualized failure rate (AFR).
Only 4 failures observed: the MTBF estimate is rough. More operating time narrows it.
Reliability and AFR assume a constant failure rate (the exponential model), which fits the flat middle of the bathtub curve but not early failures or wear-out.
| Hours | Reliability % | Failed % |
|---|---|---|
| 100 | 96.0789 | 3.9211 |
| 1,000 | 67.032 | 32.968 |
| 8,760 | 3.0077 | 96.9923 |
| 26,280 | 0.0027 | 99.9973 |
| 43,800 | 0 | 100 |
| 87,600 | 0 | 100 |
MTBF (mean time between failures) is the average operating time between failures of a repairable system. This calculator works it out from field data — units, hours and failures — or starts from a datasheet MTBF, and turns it into the figures people actually use: failure rate and FIT, MTTR, availability, the chance of surviving a mission time and the annualized failure rate (AFR) that drive makers and Backblaze report.
How to use this tool
- Choose field data or a rated MTBF.
- For field data, enter the number of units, the hours each ran, the failures and the total repair time.
- For a rated MTBF, enter it and the mean time to repair if you know it.
- Set a mission time (one year of 24/7 use is 8,760 hours) and read the results.
Formula
MTBF = total operating hours ÷ failures; λ = 1 ÷ MTBF; FIT = λ × 10⁹; A = MTBF ÷ (MTBF + MTTR); R(t) = e^(−t ÷ MTBF); AFR = 1 − e^(−8760 ÷ MTBF)
MTTR is total repair time ÷ failures. The observed AFR used by Backblaze is failures ÷ (drive-days ÷ 365), which equals failures ÷ unit-years. R(t) and the MTBF-based AFR assume a constant failure rate. Change "powered-on hours per year" for equipment that is not on all the time.
Example
Ten servers running 1,000 hours each give 10,000 operating hours. With 4 failures the MTBF is 10,000 ÷ 4 = 2,500 h and λ = 0.0004 per hour (400,000 FIT). If the four repairs took 20 hours, MTTR is 5 h and availability is 2,500 ÷ 2,505 = 99.80%. A datasheet MTBF of 1,000,000 h corresponds to an AFR of 1 − e^(−0.00876) = 0.87%.
Frequently asked questions
Is MTBF the same as lifespan?
No. MTBF describes the random-failure rate of a large population during its useful life. A product can have a 1,000,000-hour MTBF and still be designed for a 5-year service life, after which wear-out failures rise.
What about zero failures?
Then MTBF is not measured, only bounded. With no failures in T hours, a common 95% lower bound is T ÷ 3. Keep collecting data or use the maker's rating.
MTBF or MTTF?
MTTF (mean time to failure) is used for parts that are replaced, not repaired, such as drives or bulbs. The arithmetic here is the same; just ignore MTTR and availability.
Sources
Last updated:
Page address with these inputs: https://kakao.mobile/tools/mtbf-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). MTBF Calculator (MTTR, Failure Rate, Availability). https://kakao.mobile/tools/mtbf-tool/
“MTBF Calculator (MTTR, Failure Rate, Availability).” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/mtbf-tool/.
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