Mean time between failures (MTBF)

MTBF is the average operating time between two failures of a repairable machine: operating hours divided by the number of failures. It does not tell you when the next failure comes.

Count only the hours the machine ran. Repairs, preventive stops, and idle hours do not belong in the number. EN 13306 calls this measure MOTBF and keeps the name MTBF for a wider one.


In the standard	Mean time between failures, MTBF (EN 13306:2017, 11.3). Mean operating time between failures, MOTBF (11.2)
German	durchschnittliche Zeit zwischen Ausfällen (MTBF), durchschnittliche Betriebszeit zwischen Ausfällen (MOTBF)
In SAP PM	MTBR, "mean time between repair", in the app for failures of technical objects
Close terms	MOTBF, when only operating time counts. MTTF, for parts that are replaced, not repaired

What is the difference between MTBF and MOTBF?

EN 13306 separates two averages. MTBF averages the whole gap from one failure to the next. That gap can hold idle hours after the repair, such as a weekend with no shift. MOTBF averages only the hours the machine ran inside each gap, and the standard uses it for repairable items.

Gulati's formula divides operating time by the number of failures. In the words of EN 13306, that is MOTBF, but Gulati calls it MTBF. We recommend this version. Idle hours show the shift plan, not the machine: the same machine looks more reliable on a five-day schedule than on a seven-day schedule. Keep the name MTBF, because that is what your reports and your colleagues say, and state on the report that the number counts operating time.

How do you calculate MTBF?

MTBF = operating time / number of failures

Operating time is the hours the machine ran. Take the required time, then subtract repairs, waiting, and preventive maintenance stops.
Failures are events. Each loss of function counts once. Failure and fault explains the difference. Gulati also counts reduced speed and quality losses, so decide what counts before you count.
Period: use the same period every time. Gulati suggests a baseline of roughly twelve months.

Your work orders hold the stops. The required time comes from the shift plan.

Example

Nordwerk, a fictional plastics plant, runs injection molding machine IMM-07 for 6,000 required hours a year. In 2025 the machine had nine failures.

Item	Hours
Required time	6,000
Time to restoration, nine failures	45
Preventive maintenance stops	36
Operating time	6,000 − 45 − 36 = 5,919

MTBF = 5,919 h / 9 failures = 658 h.

Divide the 8,760 hours of the calendar year by nine failures instead and you get 973 h. That is roughly what the wider EN 13306 MTBF gives. The machine did not change, and the number is almost 50 percent higher.

MTBF says how often a machine fails. MTTR says how long it stays down. Together they give availability, and that page shows how. Use the same definition of failure for both.

What is a good MTBF?

We found no MTBF value that holds across machines. We searched EN 13306, Gulati, Wireman, and Liebstückel. Gulati's values are worked examples, not benchmarks. Compare a machine with its own history, and compare by failure mode.

When does MTBF mislead?

It does not predict the next failure. MTBF is an average. Two gaps of 100 h and 1,200 h average 650 h, and neither is near 650. If the failure rate stays constant, a machine runs a full MTBF without failing only about 37 percent of the time (Gulati, chapter 6). EN 13306 also names an early failure period and a wear-out period, where the rate is not constant.

It needs a repairable machine. A bulb has no gap between failures, because you replace it. For such parts, EN 13306 uses operating time to failure (9.17), and Gulati uses MTTF. EN 13306 does not define MTTF.

It moves with few failures. One failure more or less changes the result a lot. The number is only as steady as the count behind it. A mix of failure modes also hides what to fix.

It moves with the definition. If one shift logs a two-minute jam and another does not, MTBF changes for reasons that have nothing to do with the machine.

Sources

EN 13306:2017, Maintenance: Maintenance terminology, CEN, 2017. Clauses 9.17 to 9.19, 11.2, 11.3, and 9.22 to 9.24 (failure periods). This page explains the standard in our own words; the standard is the authority. Buy it as DIN EN 13306:2018-02 from DIN Media.
Ramesh Gulati, Maintenance and Reliability Best Practices, 3rd ed., Industrial Press, 2021, chapter 6 (MTBF formula, repairable and nonrepairable assets, reliability at constant failure rate, baseline period).
Terry Wireman, Benchmarking Best Practices in Maintenance, Reliability and Asset Management, 3rd ed., Industrial Press, 2015. Checked for an MTBF benchmark; it gives none.
Karl Liebstückel, Instandhaltung mit SAP S/4HANA, 6th ed., SAP PRESS, 2023, chapter 9.1 (MTBR in the failure analysis app). Checked for an MTBF benchmark; it gives none.