MTBF and MTTR Worksheet
MTBF tells you how long an asset runs between failures. MTTR tells you how long it takes to get it running again. This worksheet calculates both for every asset from three simple inputs and combines them into inherent availability, with a fleet total at the bottom.
| Asset | Area | Failures | Operating hours | Total repair hours | MTBF (h) | MTTR (h) | Inherent availability | |
|---|---|---|---|---|---|---|---|---|
230 hrs | 4 hrs | 98.5% | ||||||
473 hrs | 6 hrs | 98.7% | ||||||
750 hrs | 4 hrs | 99.5% | ||||||
86 hrs | 2 hrs | 97.7% | ||||||
1,700 hrs | 12 hrs | 99.3% | ||||||
183 hrs | 2 hrs | 99% |
Rows with zero failures show MTBF as 0 because the value is undefined; treat them as no failures observed, not as poor reliability. Use a long enough period that every important asset has at least a few failures. Pull failure counts and repair hours from maintenance work orders for the same period.
Get MTBF and MTTR from your maintenance data
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How to use the worksheet
List each asset and enter three numbers for the same period: the count of failures, the hours the asset actually operated and the total active repair hours. MTBF is operating hours divided by failures. MTTR is repair hours divided by failures. Inherent availability is MTBF over MTBF plus MTTR. The totals row computes fleet figures from sums, which is the right way to aggregate because it weights assets by how often they fail. Averaging the individual MTBF values would give the same weight to an asset that failed once and one that failed fifteen times.
Definitions that matter
Define a failure before you count one. Most sites use an unplanned stop that needs a corrective work order, and exclude planned maintenance, changeovers and operator adjustments. Operating hours should come from run time, not calendar time, so an asset that is idle for lack of orders does not look reliable. MTTR here is active repair time, which is different from downtime. Waiting for parts, waiting for a technician and administrative delay are separate lost time. Splitting them shows whether the answer is a better machine, a faster fix or a better spares policy.
- MTBF = operating hours / number of failures.
- MTTR = total repair hours / number of failures.
- Inherent availability = MTBF / (MTBF + MTTR).
Reading the example
In the sample, the case packer fails 14 times in about 1,200 operating hours, an MTBF near 86 hours, while the air compressor failed once. The packer's MTTR of about two hours looks acceptable, so the problem is frequency, which points to root cause analysis, not faster repair. Another asset might have rare failures but long repairs, where better spares or a trained second technician help more. Sort by MTBF to find the repeat offenders and by MTTR to find slow recoveries, then pick the action that matches.
Common mistakes
Common errors are using calendar hours instead of operating hours, mixing planned and unplanned work orders, and calculating MTBF on too few failures. With one or two events in the period the number is noise. Extend the window or group similar assets. Also remember that MTBF assumes a roughly constant failure rate, which is a reasonable approximation for many repairable systems but not for parts with wear out. In ERPs such as SAP PM, Infor EAM, Oracle EAM or Epicor, failure codes on notifications and work orders make this data far cleaner.
Frequently Asked Questions
What is the difference between MTBF and MTTR?
MTBF is the average operating time between failures of a repairable asset. MTTR is the average time to repair it after a failure. Together they determine how often an asset is lost and how long it stays out.
How do you calculate availability from MTBF and MTTR?
Inherent availability equals MTBF divided by the sum of MTBF and MTTR. It reflects failures and repairs only, ignoring planned maintenance and logistics delay, so it will be higher than the availability you measure on the floor.
Should planned maintenance count as a failure?
No. Count only unplanned events that stop the asset. Planned maintenance is scheduled and does not reflect inherent reliability. Track it separately so you can see how planned work affects availability.
How many failures do I need for a reliable MTBF?
More is better. With fewer than about five failures the figure swings widely from one period to the next. Use a longer period, or pool identical assets, before drawing conclusions about reliability.
If failure counts and repair hours already sit in your ERP maintenance module, Netray can build a read-only MTBF and MTTR report over that data. This worksheet works fine on its own.
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