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MTBF and MTTR calculator
MTBF stands for mean time between failures: the average time a repairable machine runs between one breakdown and the next. It is total operating time divided by the number of failures. MTTR, mean time to repair, is total repair time divided by the number of repairs. Together they give availability: MTBF ÷ (MTBF + MTTR).
MTBF and MTTR formulas
MTBFequalsTotal operating time divided by Number of failures
MTTRequalsTotal repair time divided by Number of repairs
AvailabilityequalsMTBF divided by MTBF + MTTR
Hours the equipment actually ran
Breakdowns in the same period
90 minutes = 1.5 hours
Usually the same as failures
Pre-filled with the worked example. Use one machine or line and one period for every figure.
Mean time between failures
200 h
Average running time between breakdowns.
Mean time to repair
4 h
Average time to restore
Availability
98.04%
MTBF ÷ (MTBF + MTTR)
Failures per 1,000 hours
5
Failure rate, 1 ÷ MTBF × 1,000
Prefer a spreadsheet? Download the Excel version: the free equipment downtime log takes stop and restart times for up to 300 stops and works out MTTR and MTBF by machine, with downtime by cause.
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How to calculate it
MTBF calculation: four numbers from the maintenance log
Operating time is the hours the machine actually ran in the period. Leave out repair time, planned stops and time with no orders, or MTBF will look better than it is.
Failures are the breakdowns that stopped the machine. Agree what counts as one, for example any stop over five minutes that needed maintenance, and keep the rule the same from month to month.
Repair time and repairs give MTTR. The count of repairs is usually the same as failures; it differs when a failure needed more than one repair or a repair is still open at the end of the period.
MTBF and MTTR point at different fixes. A low MTBF calls for preventive maintenance and operator checks that catch problems early; a high MTTR calls for faster diagnosis, spare parts on hand and standard repair procedures. Both cut unplanned downtime.
MTBF calculation example
Illustrative numbers, not a benchmarkA machine that ran 1,000 hours in a period, broke down 5 times and took 20 hours in total to repair over those 5 repairs.
Step 1
MTBF = 1,000 h ÷ 5 failures = 200 h.Step 2
MTTR = 20 h ÷ 5 repairs = 4 h.Step 3
Availability = 200 ÷ (200 + 4) = 200 ÷ 204 = 98.04%.Step 4
Failure rate = 5 ÷ 1,000 h = 5 failures per 1,000 hours.
MTBF vs MTTR vs MTTF
Three letters apart, three different questions. Pick the one that matches the part you are looking at.
| Metric | Answers | Use it for |
|---|---|---|
| MTBF | How long does it run between breakdowns? | Repairable machines: presses, fillers, conveyors |
| MTTR | How long to get it running again? | The repair process: diagnosis, parts, skills |
| MTTF | How long does a part last before it is replaced? | Parts that are replaced, not repaired: bearings, belts, bulbs |
MTTF example. Three bearings ran 4,000, 5,200 and 4,800 hours and were then replaced. MTTF = (4,000 + 5,200 + 4,800) ÷ 3 = 14,000 ÷ 3 = 4,667 hours.
Two conventions for MTBF. This calculator counts running time only: 1,000 hours ÷ 5 failures = 200 hours. Some sources measure from the start of one failure to the start of the next, which adds the repair: 200 + 4 = 204 hours. Both are in use, so write down which one you chose and keep it.
MTBF from a failure log
The example above comes from five runs. Write them down and the average is easy to check.
| Failure | Ran before it (h) | Repair (h) |
|---|---|---|
| 1 | 180 | 3.0 |
| 2 | 240 | 5.5 |
| 3 | 150 | 4.0 |
| 4 | 260 | 4.5 |
| 5 | 170 | 3.0 |
| Total | 1,000 | 20.0 |
1,000 ÷ 5 = 200 hours MTBF and 20 ÷ 5 = 4 hours MTTR. The runs range from 150 to 260 hours, so look at the spread as well as the average: one 150 hour run is a failure to investigate.
Several machines. Add the hours and the failures first, then divide. Machine A ran 1,000 hours with 1 failure and machine B ran 1,000 hours with 9. Pooled, MTBF = 2,000 ÷ 10 = 200 hours. Averaging the two machines' MTBFs (1,000 and 111) gives 556 hours and hides the problem machine.
What an MTBF of 200 hours really means
MTBF is an average, not a promise. It is the inverse of the failure rate: 1 ÷ 200 = 0.005 failures an hour, or 5 per 1,000 hours.
If failures are random, the chance of running t hours without one is e−t ÷ MTBF. That holds only while the failure rate is roughly constant. Parts that wear out do not follow it. With an MTBF of 200 hours:
- 24 hours (one day of running): e−0.12 = 89% chance of no failure.
- 100 hours: 61%.
- 200 hours, the MTBF itself: e−1 = 37%.
- 400 hours: 14%.
So 1 − 0.37 = about 63% of runs end before the MTBF is reached. An MTBF of 200 hours does not mean the machine lasts 200 hours. It means that, over a long period, you can plan for about one breakdown every 200 hours of running.
Which lever pays: MTBF or MTTR?
Availability = MTBF ÷ (MTBF + MTTR), so the same percentage can come from either side. Starting from the example (MTBF 200 hours, MTTR 4 hours, availability 98.04%):
- Halve MTTR to 2 hours: 200 ÷ 202 = 99.01%.
- Double MTBF to 400 hours: 400 ÷ 404 = 99.01%.
- Cut MTTR to 1 hour: 200 ÷ 201 = 99.50%.
Halving repair time and doubling run time give the same availability. Which one costs less depends on the machine. Compare a spare part on the shelf, a standard repair procedure and a clear call-out rule with a project to find and remove the cause of the repeat failures.
Mistakes that distort MTBF
- Counting idle time as operating time. A machine that sat 300 hours with no orders did not run 1,000 hours.
- Changing what counts as a failure. If the threshold moves from any stop to stops over five minutes, MTBF jumps without any change in the machine.
- Averaging machine MTBFs. Pool the hours and failures instead, as above.
- Using it on replaced parts. A bearing is not repaired, so use MTTF.
- Reading too much into two failures. A short log gives a number that moves with every new breakdown.
- Treating it as a guarantee. A supplier's MTBF is a statistical average under stated conditions, not the life of your machine in your plant.
Log the stops first with an equipment downtime log, then find the cause of the repeat ones with a breakdown analysis report. To put a price on the stops, use the downtime cost calculator.
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<p style="font:13px/1.4 sans-serif"><a href="https://www.theleansuite.com/tools/mtbf-mttr-calculator">MTBF and MTTR calculator</a> by LeanSuite</p>How LeanSuite helps
LeanSuite's Autonomous Maintenance has operators run photo-guided CILR checks on a tablet and tag any abnormality they find, so maintenance is alerted and can schedule the fix before the part fails in production.
See Autonomous MaintenanceRead more
- Free template: Equipment downtime log (MTTR and MTBF)
- Free template: Breakdown analysis report
- Free template: Critical spare parts list and stocking levels
- Free template: Maintenance KPI tracker
- Mean time between failures (MTBF) in the lean glossary
- Maintenance KPIs: What to Measure and Why
- Preventive maintenance in the lean glossary
- Unplanned downtime in the lean glossary
- Reliability-centered maintenance (RCM) guide
- Preventive vs Predictive Maintenance: Key Differences
- Autonomous Maintenance: The Seven-Step Process
- Reducing machine downtime: a worked month from one downtime log
- Maintenance strategy selector
- Repair or replace calculator
- OEE calculator: where availability fits in the OEE number
- Maintenance KPIs: What to Track and Why
- Root Cause Analysis for Equipment Failures: Methods and Framework
- Four Most Common Causes of Equipment Failure
- Preventive maintenance: the types, a PM schedule example and PM compliance
FAQ
MTBF and MTTR calculator: common questions
MTBF stands for mean time between failures, sometimes written mean time between failure. It is the average time a repairable machine runs between one breakdown and the next. The same letters are used for the number you get by dividing running time by the count of failures.
MTBF = total operating time ÷ number of failures. For a machine that ran 1,000 hours and failed 5 times, MTBF = 1,000 ÷ 5 = 200 hours. The failure rate is the inverse: 1 ÷ 200 = 0.005 failures per hour, or 5 failures per 1,000 hours.
Yes, for the same machine and the same definition of a failure. A higher MTBF means longer runs between breakdowns. Compare it only against that machine's own history or against identical machines counted the same way, because two plants that count failures differently will get numbers that cannot be compared.
No. MTBF is the average run between failures of a machine that is repaired and restarted. It says nothing about when the machine wears out. If failures are random, a machine with an MTBF of 200 hours has only about a 37% chance of running 200 hours without a breakdown, so most runs end sooner than the MTBF.
MTBF measures how long a machine runs between failures, so it describes reliability. MTTR measures how long it takes to get the machine running again after a failure, so it describes how quickly you can repair it. Raising MTBF means fewer breakdowns, and cutting MTTR means shorter ones.
It depends on the source. This calculator counts only running time, so a machine that ran 1,000 hours with 5 failures has an MTBF of 200 hours. Some sources measure from the start of one failure to the start of the next, which adds the repair time and gives 204 hours with a 4 hour MTTR. Both are in use, so state which one you used.
The more the better. An MTBF built from two failures can move a lot with one more, so use a longer period or pool identical machines until you have at least several failures. Add the operating hours and the failures of all the machines first, then divide, rather than averaging each machine's MTBF.
Divide the total operating time by the number of failures in the same period. A machine that ran 1,000 hours with 5 failures has an MTBF of 1,000 ÷ 5 = 200 hours. Count only the time the machine was running, not repair time or time it sat idle with no orders.
Divide the total repair time by the number of repairs. If 5 repairs took 20 hours in total, MTTR is 20 ÷ 5 = 4 hours. Decide what the repair clock includes, such as waiting for a technician or parts, and use the same rule every period so the trend means something.
Availability = MTBF ÷ (MTBF + MTTR). With an MTBF of 200 hours and an MTTR of 4 hours, availability is 200 ÷ 204 = 98.04%. This figure counts only failures and repairs, not planned stops or changeovers, so it is usually higher than the availability factor in OEE.
There is no single good number: MTBF depends heavily on the type of equipment, how hard it runs and how failures are counted. The most useful comparison is the same machine's MTBF over time. A rising MTBF shows the equipment is becoming more reliable, and a falling one is an early sign of a developing problem.
MTBF, mean time between failures, applies to repairable equipment that is fixed and put back into service. MTTF, mean time to failure, applies to parts that are replaced rather than repaired, such as bearings or bulbs, and measures how long they last on average. Using the wrong one gives a number that answers the wrong question.
Then MTBF cannot be calculated, because there is nothing to divide by. All you can say is that it is longer than the operating time. The calculator says so instead of showing a number; a longer period, or several similar machines together, gives a real figure.
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