MTTR (Mean Time To Repair) is the mean time to repair: the total time spent on repairs divided by the number of failures in the period. MTBF (Mean Time Between Failures) is the mean time between failures: the total operating time divided by the number of failures in the same period. The two indicators answer different questions, and that is exactly why they are so often confused: MTTR measures how fast the team returns the equipment to operation, while MTBF measures the reliability of the asset, that is, how long it runs between one breakdown and the next.
In this guide you will find the formulas of both indicators written out in words, a step-by-step calculation example with numbers, the difference from MTTF (the most common confusion of the trio) and a free spreadsheet to calculate everything automatically.
What is MTTR (mean time to repair)
MTTR measures maintainability: the ease and speed with which a piece of equipment returns to operation after a failure. It is the average of how long each repair consumed in the analyzed period.
MTTR = Total repair time ÷ Number of failures in the period
In words: add up all the hours of unavailability caused by corrective repairs in the period and divide by the number of failures handled in that same period. The result comes out in hours per failure.
Two cautions define whether your MTTR is reliable:
- What goes into the calculation: repair time starts when the equipment stops and ends when it returns to operation. This includes diagnosis, waiting for a part, waiting for release and the intervention itself. A high MTTR does not always mean a slow team: often the wrench time is short and what weighs is the waiting. This detail changes how you read the indicator, and we explain it in depth in how waiting inflates the MTTR of work orders.
- Corrective only: planned stoppages (preventive, predictive, improvements) do not enter the calculation. Mixing the two types of stoppage distorts the indicator and hides the real problem.
The lower the MTTR, the smaller the impact of each failure on production. If your number is high and you want a structured action plan, see the practical guide to reducing MTTR in maintenance.
What is MTBF (mean time between failures)
MTBF measures the reliability of a repairable piece of equipment: on average, how long it operates between one failure and the next. It is the indicator that tells you whether breakdowns are becoming rarer or more frequent.
MTBF = Total operating time ÷ Number of failures in the period
In words: take the time the equipment was actually operating in the period (subtracting the time stopped in repair) and divide by the number of failures that occurred. The result comes out in operating hours per failure.
Points of attention for the calculation to be correct:
- Operating time, not calendar time: if the equipment runs on an 8-hour shift, a month has about 176 operating hours, not 720. Using the whole calendar inflates the MTBF and creates a reliability that does not exist.
- A failure is a failure: the MTBF considers unplanned stoppages. A scheduled preventive stoppage does not count as a failure.
- Analyze per equipment or family: a plant-wide average MTBF hides the villains. The indicator becomes useful when stratified by tag or equipment family, pointing to where maintenance engineering should investigate chronic failures.
The MTBF is only as good as the failure record that feeds it. We show this dependency in detail in the article on MTBF and the quality of execution data.
MTTF: the third indicator that causes the confusion
MTTF (Mean Time To Failure) is the mean time to failure, and this is where most texts go wrong: MTTF applies to non-repairable items, those that are replaced when they fail, such as lamps, fuses, bearings discarded after breaking or electronic components. MTBF applies to repairable items, which return to operation after the fix, such as pumps, motors and gearboxes.
MTTF = Total operating time of a set of items ÷ Number of items that failed
In practice, the difference shows up like this: a centrifugal pump has an MTBF, because it breaks, is repaired and returns to operation. The bearing inside it has an MTTF, because when it fails it is replaced by a new one. A common mistake is to use the catalog MTBF of a component as if it were the MTBF of the whole equipment: they are different quantities, measured on different populations.
| Indicator | What it measures | Applies to | Formula in words |
|---|---|---|---|
| MTTR | Repair speed (maintainability) | Repairable items | Total repair time divided by the number of failures |
| MTBF | Time between failures (reliability) | Repairable items | Total operating time divided by the number of failures |
| MTTF | Mean life until failure | Non-repairable items | Total operating time divided by the number of items that failed |
Step-by-step numerical example
Let us calculate the MTTR and MTBF of a centrifugal pump (tag P-101) running continuously, using a 30-day month as the analysis period. The numbers are hypothetical, built to make the mechanics of the calculation clear.
Step 1: gather the period data.
- Analysis period: 30 days of continuous operation, that is, 720 calendar hours.
- Failures in the period: 3.
- Duration of each repair: 4 hours, 6 hours and 2 hours, adding up to 12 repair hours.
Step 2: calculate the MTTR.
MTTR = 12 repair hours ÷ 3 failures = 4 hours. On average, each failure of P-101 consumes 4 hours until the equipment returns to operation.
Step 3: calculate the operating time.
Operating time = 720 calendar hours - 12 hours stopped in repair = 708 hours. Here we are assuming there were no other stoppages in the month; if there had been preventive or operational stoppages, they would also come out of the operating time (but would not count as a failure).
Step 4: calculate the MTBF.
MTBF = 708 operating hours ÷ 3 failures = 236 hours. On average, P-101 operates 236 hours (almost 10 days) between one failure and the next.
Step 5: read the result together.
P-101 breaks every 236 hours and takes 4 hours to return. If the MTBF drops over the coming months, failures are becoming more frequent and the maintenance plan needs to be reviewed. If the MTTR rises, the repair process (diagnosis, parts, waiting) is getting worse, even if the equipment keeps breaking at the same frequency.
Availability: the indicator that unites MTTR and MTBF
With both indicators in hand, you calculate the intrinsic availability of the equipment, the probability of finding it ready to operate:
Availability = MTBF ÷ (MTBF + MTTR) × 100
In the P-101 example: 236 ÷ (236 + 4) × 100 = 98.3%. For every 100 hours production needs the pump, it is available for just over 98.
The formula makes the double lever of maintenance clear: availability rises when the MTBF grows (fewer failures) or when the MTTR drops (faster repairs). To understand how this indicator unfolds in asset management, see the article on asset availability and why it matters for maintenance.
How to interpret the results (and why there is no universal range)
The most common question after the calculation is: "is my MTBF good?". The honest answer is that there is no single valid range for the whole industry. The MTBF of a chemical process pump, an overhead crane and a filling machine are incomparable quantities: they depend on the equipment design, the operating regime, the environment and the criticality. Any generic table of "ideal MTBF" by sector should be read with suspicion.
PM Run practical recommendation is to interpret both indicators on three axes:
- Trend before absolute value: a rising MTBF and a stable or falling MTTR is the sign of a maintenance that is working. The movement of the indicator over the months says more than the isolated number of a single month.
- Compare the equipment with itself and with its siblings: the fair comparison is between this year P-101 and last year P-101, or between pumps of the same family in the same plant. That is how the villains appear.
- Cross with criticality: a low MTBF on a class C asset may be tolerable (conscious corrective). The same number on a class A asset calls for an immediate action plan.
These indicators do not live alone: they make up the management panel together with backlog, availability and cost. To see the full picture, check the guide to the 12 maintenance indicators. And to understand how to structure the maintenance planning that sustains this measurement, we invited a specialist to speak about the subject in this video.
Where the data comes from in SAP PM
For those who run maintenance in SAP PM, MTTR and MTBF do not need to be born in a parallel spreadsheet: the system itself calculates both, as long as the recording discipline exists. The chain is this:
- Malfunction notification: it is the origin of the data. By recording the notification with the breakdown indicator marked and the start and end dates and times of the malfunction filled in, the system starts to know when the equipment stopped and when it came back.
- Maintenance order: linked to the notification, it carries the intervention, the materials and the hours worked reported in the confirmations.
- Analysis in the information system: with the notifications well filled in, the module malfunction analysis transactions (such as MCJB, per equipment) deliver MTTR and MTBF calculated by tag, with no manual math.
The fragile link of the chain is human: a notification opened without a real start time, a malfunction closed days after the repair, a stoppage not signaled. The system calculates with what it receives; if the field lies, the transaction returns a lying indicator. That is why the quality of reporting at the source is worth more than any sophisticated report at the output.
How to set targets for MTTR and MTBF
Without a universal range, a well-built target is relative to your own baseline. The practical path we recommend:
- Measure 3 to 6 months before setting a target: establish the baseline per equipment family with data that is already reliable. A target on bad data is a lottery.
- A percentage-improvement target, not an absolute value: "reduce the MTTR of the pump family by 15% in two quarters" is auditable and fair; "MTTR under 2 hours" copied from another plant is not.
- Prioritize by impact: start with class A assets with the worst MTBF and MTTR pair. Improving 10% where it hurts is worth more than 30% on a redundant asset.
What makes the calculation go wrong
MTTR and MTBF are simple fractions. The risk is not in the math, it is in the data that goes into it:
- Late or from-memory reporting: when the technician records the intervention hours or days later, the start and end times become an estimate. The calculated MTTR then measures the team memory, not the repair.
- Failure without a record: small stoppages solved "on the fly", without a work order, make the MTBF look better than it is. The equipment breaks, but the system does not find out.
- Everything classified as corrective (or nothing): without discipline in classifying the stoppage type, preventive work contaminates the MTTR and the MTBF loses its meaning.
- Periods that are too short: calculating MTBF with a single failure in the period generates unstable numbers. Prefer moving windows (3, 6, 12 months) for equipment with few failures.
The common root of the four problems is the same: the quality of reporting at the source. When the record happens at the moment and place of execution, the indicators reflect the real plant.
MTTR and MTBF spreadsheet for download
To apply the calculation in your plant today, download PM Run free spreadsheet. It brings a stoppage log tab and an automatic calculation tab: you enter the stoppages with start and end, inform the operating time of each piece of equipment and receive MTTR, MTBF and availability calculated by tag, with the example from this article already filled in.
Download the MTTR and MTBF calculation spreadsheet here (Excel, free).
Frequently Asked Questions
What is the difference between MTBF, MTTR and MTTF?
MTBF is the mean time between failures of a repairable piece of equipment and measures reliability. MTTR is the mean time to repair and measures how fast the asset returns to operation. MTTF is the mean time to failure of non-repairable items, those replaced when they break, such as bearings and electronic components.
How do you calculate the MTTR?
Add up all the stoppage hours caused by corrective repairs in the period and divide by the number of failures handled. If a piece of equipment had 3 failures that added up to 12 repair hours, the MTTR is 4 hours. Planned stoppages, such as preventive work, stay out of the calculation.
How do you calculate the MTBF?
Divide the total operating time of the equipment in the period by the number of failures that occurred. The operating time subtracts the hours stopped in repair. A piece of equipment that operated 708 hours and failed 3 times has an MTBF of 236 operating hours between failures.
What is a good MTBF?
There is no universal range: the MTBF depends on the type of equipment, the operating regime and the criticality. The correct reading is to follow the trend (a rising MTBF indicates effective maintenance), compare the equipment with its own history and with equipment of the same family in the plant.
What is the relationship between MTTR, MTBF and availability?
Intrinsic availability is calculated as MTBF divided by the sum of MTBF and MTTR, multiplied by 100. In other words, availability rises through two paths: increasing the time between failures (higher MTBF) or reducing the time of each repair (lower MTTR).
Measure the real MTTR with reporting at the moment of execution
The whole calculation in this guide depends on one thing: stoppage and return times recorded with precision. PM Run Mobility takes reporting to the technician mobile device, integrated with SAP PM in real time and working even in areas without connection, so that your plant MTTR and MTBF come from the real execution, not from the memory at the end of the shift. Get to know PM Run Mobility and see how to measure your team real MTTR.
