Mean time between failures (MTBF) is the average operating time a repairable piece of equipment runs between one failure and the next. A higher MTBF means the equipment fails less often.
It pairs with mean time to repair. MTBF tells you how often something breaks. MTTR tells you how long it stays broken.
How to calculate MTBF
MTBF = Total operating time ÷ Number of failures
Say a customer has 10 identical units that each ran 2,000 hours over the year, for 20,000 operating hours in total, and between them they had 8 failures. MTBF is 20,000 ÷ 8, or 2,500 hours.
Use operating time, not calendar time, when you have it. A unit that runs only in summer should not get credit for the months it sat idle. If you only have calendar time, say so in the report.
Count only failures, meaning unplanned stoppages that needed a repair. Planned maintenance visits are not failures and should not be in the count.
Why it matters
MTBF is one of the few ways to test whether your preventive maintenance is doing anything. If MTBF on a group of units rises after you start a PM program, the visits are probably preventing failures. If it stays flat, you may be servicing the wrong things or at the wrong interval.
It also helps you plan capacity. If you know a fleet of equipment fails on average every 2,500 operating hours, you can estimate how many reactive calls to expect from that customer each season. That feeds capacity planning and tells you how much room to hold on the board for urgent work. The field service reporting guide covers building these reports from work order data.
To calculate MTBF at all, your service history needs to tie each failure to a specific unit. Without that, you only know how many calls a site produced.
A common mistake
Reading MTBF as a prediction for a single unit. An MTBF of 2,500 hours does not mean each unit will run 2,500 hours and then fail. It is an average across many units and many failures. Some will fail much sooner and some much later.
Related terms
- Mean time to repair measures how long each failure lasts.
- Asset management holds the per-unit records MTBF needs.
- Predictive maintenance tries to act before the next failure.
Common questions
What is the difference between MTBF and MTTF?
MTBF applies to equipment you repair and return to service. Mean time to failure (MTTF) applies to items you replace rather than repair, such as many parts and components.
Can a small service company calculate MTBF?
Yes, if each work order records the specific unit serviced and whether the visit was a failure or planned work. The arithmetic is simple; the record keeping is the hard part.
Is a higher MTBF always better?
For the equipment owner, yes. For your business it means fewer reactive calls on that equipment, which is why many contracts pair maintenance with MTBF reporting.