The formulas
Uptime is the time period minus repair downtime. Using the full calendar period instead of uptime is the most common MTBF mistake — it inflates the number, and the more downtime an asset has, the worse the error gets.
What MTBF does not mean
An MTBF of 1,000 hours does not mean the asset runs for 1,000 hours and then fails. It's an average rate. If failures are random (the usual assumption behind published MTBF figures), only about 37% of units make it to the MTBF without a failure. That's why this calculator reports mission reliability separately: the odds of getting through a specific stretch — a shift, a week, a campaign — without a breakdown.
That random-failure assumption breaks down for wear-out items like bearings, belts, and seals, where failures cluster late in life. For those, MTBF is still a useful trend number, but a Weibull analysis tells you more about when to intervene.
MTBF vs. MTTF
MTBF is for repairable equipment: pumps, motors, conveyors. MTTF (mean time to failure) is for things you replace rather than repair: fuses, bulbs, many electronic parts. The math is the same idea; the difference is whether there's a "between."
Which "R" is your MTTR?
MTTR gets used for repair, restore, recover, and respond, and they aren't the same number. Mean time to repair is often hands-on wrench time. Mean time to restore runs from failure until the asset is back in service, including waiting on techs and parts. This calculator uses failure-to-back-in-service, because that's what drives availability. Whatever you choose, keep it consistent month to month.
Inherent vs. operational availability
Inherent availability counts only active repair time. It's a design number: what the equipment could achieve with instant access to parts and people. Operational availability includes the real-world delays: waiting on parts, permits, crews, and often preventive maintenance downtime too. The gap between the two is usually where the improvement opportunity is, and it's mostly a planning and stores problem, not an equipment problem.
FAQ
What is a good MTBF?
There's no universal number — it depends entirely on the asset class, duty cycle, and operating context. The useful comparison is an asset against its own history, or against identical assets at your site. A falling MTBF is the signal that matters.
Should planned downtime count against MTBF or availability?
Planned PM work isn't a failure, so it doesn't count toward MTBF. For availability, it depends which one you're reporting: inherent availability leaves it out, while operational availability usually includes it. Label which you're using.
Why does the failure log mode want a period start and end?
MTBF needs total operating time, and a failure list alone doesn't contain it — you also need to know how long the asset ran without failing. If you leave the period blank, the calculator uses the first failure to the last restore, which understates MTBF. It will tell you when it does this.
How do I export failures from my CMMS?
Pull corrective or breakdown work orders for the period with the asset number, the reported or failure date and time, and the actual finish or return-to-service time. Copy those three columns from Excel and paste them in. Column names don't matter — only the order.
Is my data uploaded anywhere?
No. The calculations run in plain JavaScript on this page. Nothing you paste leaves your browser.
Sources & standards
- NIST/SEMATECH e-Handbook of Statistical Methods, Chapter 8, “Assessing Product Reliability.” — MTBF, failure rate, and the exponential reliability model. itl.nist.gov
- U.S. Department of Defense. Reliability, Availability, Maintainability, and Cost Rationale Report Manual, 2009. — Definitions of inherent and operational availability.
Formulas on this page are standard reliability engineering definitions. There’s no universal “good” MTBF or availability — compare an asset against its own history or identical assets.