How to calculate MTBF, MTTR and availability
All three metrics come from three numbers for the period: how long the equipment should have run, how many times it broke down and how long those breakdowns kept it stopped.
MTBF = (scheduled time − repair time) ÷ number of failures
MTTR = total repair time ÷ number of failures
Availability = MTBF ÷ (MTBF + MTTR)
| Term | What it is |
|---|---|
| Scheduled time | the hours the equipment should run in the period (working shifts, not 24 hours a day) |
| Repair time | downtime caused by failures, from breakdown to release, including waiting for a technician and parts |
| Number of failures | breakdowns only. Preventive maintenance and changeovers are not failures |
Availability can also be computed as operating time ÷ scheduled time. Both give the same result.
Worked example
The values already in the calculator are for 3 assets in a month of 22 working days with two 8-hour shifts:
- Scheduled hours per asset: 22 × 16 = 352 h. For all three: 1,056 h
- Failures in the month: 6
- Total repair time: 13 h
Now the math:
- Operating time = 1,056 − 13 = 1,043 h
- MTBF = 1,043 ÷ 6 = 173.8 h: on average, one breakdown every 174 operating hours
- MTTR = 13 ÷ 6 = 2.17 h, about 2 h 10 min per failure
- Availability = 173.8 ÷ (173.8 + 2.17) = 98.77%
If you paste the breakdown list instead, you get the same numbers split per asset. In the example, Press 1 broke down 3 times while Lathe 2 stayed down longer each time. Those are different problems: the press needs its failure cause removed, the lathe needs faster repairs.
How to read the result
- Low MTBF means the equipment breaks down often. Find which failures repeat (a Pareto chart of causes helps) and remove the root cause with preventive maintenance or an improvement.
- High MTTR means each breakdown takes long to fix. Look at where the time goes: waiting for a technician, diagnosis, missing spare parts, or the repair itself.
- Availability combines both. It is also the first factor of OEE, so better MTBF and MTTR raise OEE directly.
The failure rate is the inverse of MTBF. The chance of running the mission time without failure uses R(t) = e^(−t ÷ MTBF), which assumes random failures at a constant rate. That is a fair approximation for equipment in its useful-life phase, but not for brand-new machines (early failures) or worn-out ones.
Common mistakes
- Using 24 hours a day as scheduled time for equipment that runs two shifts. MTBF gets inflated and availability looks better than it is.
- Counting preventive maintenance and changeovers as failures. MTBF measures breakdowns. Planned stops belong to other metrics.
- Measuring MTTR as wrench time only. For production, the machine is down from the moment it breaks, so waiting for the technician and the part is part of MTTR.
- Looking only at the fleet average. One bad machine hides in the average. Compare asset by asset.