MTBF and MTTR Explained Simply: How to Measure Your Plant’s Reliability

Monday morning, the daily briefing. Production management asks why the filling line was down so often last month. Maintenance replies: “Really just a few minor faults.” The shift supervisor says: “It felt like every other day.” Both are right in their own way – and neither can prove it.

This is exactly where two metrics help: MTBF and MTTR. One shows how often a machine fails. The other shows how long it takes until it runs again. Together, they turn a gut feeling into a solid statement.

In this article you will learn how to calculate MTBF and MTTR correctly, how the two values relate to availability and OEE, and which typical mistakes make the figures worthless. A fictitious worked example takes you through it step by step.

What do MTBF and MTTR mean?

MTBF stands for “mean time between failures”, i.e. the average operating time between two failures. The metric describes how long a repairable machine runs on average before it fails again. The higher the MTBF, the more reliable the machine.

MTTR stands for “mean time to repair” or – depending on the source – “mean time to restoration”. It means the average time until a failed machine is working again. The lower the MTTR, the faster your maintenance team responds and the shorter the downtime.

Maintenance terminology is defined in EN 13306. It distinguishes between the pure repair time and the total time until restoration. In practice, this means: define internally and bindingly which time components you include in the MTTR. Otherwise you will be comparing apples with oranges.

The difference: MTTF

For parts that are not repaired but replaced, such as a ball bearing, a fuse or a relay, you use MTTF (“mean time to failure”): the average time until the first failure. MTBF applies to repairable systems such as a machine, a pump or an entire line.

The formulas: how to calculate MTBF and MTTR

Both metrics are simple averages. The only important thing is to calculate for the same period and the same machine.

• MTBF = actual operating time ÷ number of failures
• MTTR = total downtime from failures ÷ number of failures
• Technical availability = MTBF ÷ (MTBF + MTTR)
• Failure rate λ = 1 ÷ MTBF (assuming a constant failure rate)

The actual operating time is the planned operating time minus the time the machine was down due to failures. Planned stoppages such as maintenance, cleaning or weekends without production count neither as operating time nor as failures.

Fictitious worked example: a filling line over one month

Take a filling line running three shifts. In the month under review, 600 hours of production are planned. Maintenance records five unplanned failures with these downtimes:

Failure	Cause	Downtime
1	Sensor on the labeller dirty	0.5 h
2	Motor protection circuit breaker on the conveyor tripped	1.0 h
3	Solenoid valve on the capping station faulty	2.5 h
4	Bearing damage on the drive, spare part not in stock	9.0 h
5	Connector worked loose	2.0 h
	Total	15.0 h

This gives:

1. Actual operating time = 600 h – 15 h = 585 h
2. MTBF = 585 h ÷ 5 = 117 h
3. MTTR = 15 h ÷ 5 = 3 h
4. Technical availability = 117 ÷ (117 + 3) = 0.975, i.e. 97.5%

At first glance that looks good. But the table reveals more than the average: a single failure – the bearing damage – causes more than half of the total downtime. And most of that was waiting for the spare part, not the repair itself.

The average tells you that there is a problem. The individual values tell you where it lies.

How MTBF and MTTR relate to OEE

OEE (overall equipment effectiveness) is the best-known metric for a machine’s productivity. It is made up of three factors:

• Availability: actual run time ÷ planned production time
• Performance: actual output ÷ theoretically possible output during the run time
• Quality: good parts ÷ parts produced

The formula is: OEE = availability × performance × quality. All three factors are entered as a fraction between 0 and 1.

Important: availability in OEE is not identical to the technical availability derived from MTBF and MTTR. OEE availability includes all stoppages that occur during planned production time – so changeovers, material shortages or missing staff too. MTBF and MTTR, by contrast, only consider technical failures.

Calculating OEE: the example continued

In the fictitious example, on top of the 15 hours of failures there are another 25 hours of format changeovers and cleaning during planned production time. So the line ran for 560 of 600 hours. Performance was 90% and the quality rate 98%.

1. Availability = 560 h ÷ 600 h ≈ 93.3%
2. OEE = 0.933 × 0.90 × 0.98 ≈ 0.82, i.e. around 82%

For maintenance, this is a useful way to put things in context: of the 40 hours of availability loss, 15 are due to technical failures. Reducing MTTR therefore improves OEE directly – without changing anything about changeover processes or quality.

What’s inside the MTTR? Breaking down the time

MTTR looks like a single number. In reality it is made up of several phases. If you want to reduce it, you need to know which phase takes the most time:

1. Reporting time: from the failure until maintenance is informed.
2. Response time: until a technician is at the machine.
3. Diagnosis: finding the cause, reading the fault memory, measuring.
4. Spare parts procurement: fetching the part from the store or – if it’s missing – sourcing it.
5. Repair: removal, installation, adjustment.
6. Restart: functional test, release, ramping up the line.

In many plants, the actual repair is the shortest phase. Most of the time is lost in diagnosis and waiting for spare parts. That is exactly where the biggest levers are.

The data basis: recording faults properly

Metrics are only as good as the data behind them. Before you report MTBF and MTTR, every fault should be recorded with the same minimum information:

• machine and assembly, ideally with a unique asset number
• start and end of the downtime, with time of day
• fault symptoms and the cause identified
• parts replaced, with manufacturer and part number
• waiting times, for example for staff or spare parts, noted separately

Whether you use a maintenance system, a spreadsheet or, to begin with, a form at the machine is secondary. What matters is that all shifts record according to the same rules.

How to improve MTBF and MTTR

Increasing MTBF: fewer failures

• Fix causes, not symptoms: investigate recurring failures with a root cause analysis, for example using the 5 Whys method or an FMEA.
• Maintain preventively: a maintenance plan with sensible intervals prevents many failures caused by wear, lack of lubrication or contamination.
• Monitor condition: vibration measurement, thermography or oil analysis reveal damage before it leads to downtime.
• Reduce operating errors: clear work instructions and training for operators.

Reducing MTTR: back up and running faster

• Stock critical spare parts: parts with a long replenishment lead time and a high failure impact belong in the store.
• Documentation to hand: make circuit diagrams, bills of materials and nameplate data available digitally at the machine.
• Maintain fault catalogues: record known faults with cause and solution, so the next shift doesn’t start searching from scratch.
• Prepare sourcing channels: for parts you don’t stock, clarify in advance where and how you can get them in an emergency.

Typical mistakes with MTBF and MTTR

• Short faults not recorded: if you only log failures lasting more than an hour, you get a flattering MTBF. Set a clear minimum duration and stick to it.
• Planned stoppages included: maintenance and cleaning are not failures. Otherwise they distort both metrics.
• Data basis too small: with two failures a year, an average says little. Look at longer periods or several similar machines.
• Only the average: a single long failure can heavily distort the MTTR. Always look at the individual values too.
• Manufacturer data misunderstood: an MTBF in a data sheet, for example for a power supply, is a statistical value under laboratory conditions. It does not mean that every single device will last that long.

When spare parts determine the MTTR

In the worked example, a missing bearing more than doubled the MTTR. That is a typical pattern: the repair itself takes an hour, the search for the right part half a day or longer. It gets particularly difficult with older machines, discontinued components or parts from foreign manufacturers.

KTB Europe is a family business from Hamburg and has been sourcing spare parts and MRO materials of all brands worldwide since 1976. You send us the part number, a photo of the nameplate or a description. If the original part is no longer available, we look for suitable alternatives. How to identify and stock critical parts systematically is explained on our page about spare parts management. For urgent cases, see our article on emergency spare parts procurement, and how to prevent failures in advance is shown in the article How to create a maintenance plan.

Looking for a specific part? Send us your enquiry via the contact page.

Conclusion

MTBF and MTTR are easy to calculate and very revealing – provided the data is right. MTBF shows how reliably a machine runs. MTTR shows how quickly your organisation responds to failures. Together with OEE, you can see what share of the losses is technical. Start with clean fault recording, break the MTTR down into its phases and tackle the areas where the most time is lost.