What MTBF and MTTR measure.
MTBF (Mean Time Between Failures) is the average operating time between two failures, and MTTR (Mean Time To Repair) the average time to restore the equipment: a machine with 120 hours of MTBF and 4 hours of MTTR is able to run 96.8% of the time.
The two metrics answer two different questions:
- MTBF measures reliability. How often the machine fails.
- MTTR measures maintainability. How fast it is back up.
Maintenance vocabulary is set by the European standard EN 13306, Maintenance terminology (current edition 2017).
Not to be confused: in IT, MTTR often means the mean time to restore a service after an incident. This page is about industrial maintenance, so about repairing equipment.
The formulas.
All three are computed over the same period, a month for instance:
- MTBF = total operating time ÷ number of failures.
- MTTR = total repair time ÷ number of failures.
- Availability = MTBF ÷ (MTBF + MTTR).
Two counting rules make the result reliable:
- What a failure is. An unplanned stop that needs an intervention. Set a threshold (say 5 minutes) to separate failures from small stops, which belong to performance in OEE.
- What MTTR counts. The repair alone, or the whole stop, waiting for the technician and the parts included. Both conventions exist. Pick one and keep it, or the curve moves while nothing changed on the shop floor.
A worked example.
One press, over a month (example figures):
| Input | Value |
|---|---|
| Operating time | 600 h |
| Number of failures | 5 |
| Total repair time | 20 h |
The calculation:
- MTBF = 600 ÷ 5 = 120 h. The press runs 120 hours on average between two failures.
- MTTR = 20 ÷ 5 = 4 h. Each failure stops it for half of an 8 hour shift on average.
- Availability = 120 ÷ (120 + 4) = 96.8%.
Calculate MTBF and MTTR.
Enter the figures of one period. The result updates as you type.
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MTBF, MTTF and MTTA.
Three close acronyms come up often:
| Acronym | Name | What it measures |
|---|---|---|
| MTBF | Mean Time Between Failures | Average operating time between two failures, for repairable equipment |
| MTTF | Mean Time To Failure | Average life before failure, for a component you replace instead of repairing |
| MTTA | Mean Time To Acknowledge | Average time between the alert and someone taking charge of the failure |
MTTA comes mostly from on-call teams. On the shop floor, it shows the time lost before the repair even starts, often hidden inside MTTR.
Reading them together.
Availability alone does not tell you which lever to pull. Two machines can show the same:
| Machine | MTBF | MTTR | Availability | Lever |
|---|---|---|---|---|
| A | 120 h | 4 h | 96.8% | Repair faster: spare parts in stock, a troubleshooting procedure |
| B | 30 h | 1 h | 96.8% | Fail less often: preventive maintenance, root cause of repeated failures |
Machine A fails rarely but for long. Machine B fails often but restarts quickly. Same result, opposite action plans.
Tracking maintenance.
MTBF and MTTR feed on work orders: date, start and end time, machine, cause. The more complete that data, the more accurate the metrics. The two main sources are a CMMS, the maintenance management software, or logs kept by the team.
Their effect also shows in the availability factor of OEE: fewer failures and shorter repairs raise it directly. A production dashboard puts these KPIs on one screen.
The gain comes when those figures are up to date without effort. That is the mechanism we built for Maeva & Co, a property management company: exports are collected, figures computed and the dashboard consolidated every 24 hours, with no manual re-entry. More than 400 hours freed up per year.
Source: reference and edition of the standard from its AFNOR listing (NF EN 13306, January 2018, French adoption of EN 13306:2017).
MTBF and MTTR: what people ask us.
How do you calculate MTBF?
Divide the operating time of the period by the number of failures. A machine that ran 600 hours in the month with 5 failures has an MTBF of 120 hours.
How do you calculate MTTR?
Divide the total time spent repairing by the number of failures. 20 hours of repair for 5 failures give an MTTR of 4 hours. Set a rule on what counts (the repair alone, or waiting for the technician and the parts as well) and keep it from one month to the next.
How do you calculate availability from MTBF and MTTR?
Availability = MTBF ÷ (MTBF + MTTR). With an MTBF of 120 hours and an MTTR of 4 hours: 120 ÷ 124 = 96.8%. It is the share of time the equipment is able to run.
What is the difference between MTBF and MTTF?
MTBF applies to repairable equipment: it measures the time between two failures. MTTF (Mean Time To Failure) applies to a component you replace instead of repairing: it measures its average life before failure.
Is IT's MTTR the same metric?
The idea is close, the content differs. In IT, MTTR often means the mean time to restore a service after an incident. In industrial maintenance, it is the mean time to repair a piece of equipment.