MTBF / MTTR Calculator

Reliability, maintainability and inherent availability from failure history

Inputs

Operating time (hours)Total time the asset was running over the period.Number of failures Total repair time (hours)Combined time spent repairing those failures.

MTBF (mean time between failures)

667h

MTTR (mean time to repair)

4.0h

Inherent availability

99.4%

Excellent (≥ 99%)

Failure rate

1.5per 1,000 h

How it's calculated

Failure history

Operating time, failures, repair time

MTBF

Operating time ÷ failures

MTTR

Repair time ÷ failures

Availability

MTBF ÷ (MTBF + MTTR)

MTBF = operating time ÷ number of failures (the average running time between breakdowns).

MTTR = total repair time ÷ number of failures (the average time to restore the asset).

Inherent availability = MTBF ÷ (MTBF + MTTR). This is the uptime you'd expect from reliability and repair time alone, before logistics and planning delays.

Failure rate = failures ÷ operating time, shown here per 1,000 operating hours.

How to calculate MTBF and MTTR: a worked example

Take a pump with a year of history, using the same figures the calculator opens with. It ran 8,000 operating hours and failed 12 times, and those repairs took 48 hours in total.

Step	Calculation	Result
MTBF	8,000 h ÷ 12 failures	667 h
MTTR	48 h ÷ 12 repairs	4.0 h
Inherent availability	667 ÷ (667 + 4.0)	99.4%
Failure rate	12 ÷ 8,000 h × 1,000	1.5 per 1,000 h

Four hours to repair looks healthy, but 12 failures a year is one a month. This pump's availability rests on quick repairs, not on reliability.

The MTBF and MTTR formulas

MTBF, mean time between failures, is operating time divided by the number of failures. It describes how long a repairable asset runs, on average, before it breaks down. MTTR, mean time to repair, is total repair time divided by the number of repairs, and describes how long each restoration takes.

Together they give inherent availability, MTBF ÷ (MTBF + MTTR). That is the uptime reliability and repair speed alone would deliver, before waiting for parts, permits or people. Achieved and operational availability add those delays back in, which is why they always come out lower.

For items that are replaced rather than repaired, such as bearings, seals or lamps, the equivalent measure is MTTF, mean time to failure.

Measure	Formula	What it tells you
MTBF	Operating time ÷ number of failures	How often the asset fails
MTTR	Total repair time ÷ number of repairs	How quickly it is restored
Inherent availability	MTBF ÷ (MTBF + MTTR)	Uptime from reliability and repair alone
Failure rate	Failures ÷ operating time	The inverse of MTBF, often quoted per 1,000 hours

Getting the inputs right

Use operating hours, not calendar hours. A crusher that runs 60 per cent of the year has a very different MTBF depending on which clock you divide by.
Count functional failures, not every work order. Inspections, adjustments and planned replacements are not failures, and including them makes an asset look far less reliable than it is.
Be consistent about where repair time starts and stops. Some sites include diagnosis and waiting for parts in MTTR, others count only hands-on time. Either works if it is applied every time and written down.
Calculate at the right level. An MTBF for a whole conveyor mixes belt, idler, drive and pulley failures that behave nothing alike. Group by equipment class and failure mode, which depends on consistent failure coding in the CMMS.
Do not read MTBF as a life expectancy. An MTBF of 667 hours does not mean each run lasts 667 hours. It is an average, and individual runs vary widely around it.

Why it matters

MTBF and MTTR pull apart the two things behind uptime. How often an asset fails, and how fast you get it back. That split matters because you fix them in completely different ways. Poor MTBF is a reliability problem, so you look at design, maintenance strategy and how the asset is run. Poor MTTR is a maintainability problem, usually sorted through spares, access, clear procedures and planning.

Track them over time and across a fleet and the picture gets useful. The bad actors eating your availability stand out, the case for reliability work writes itself, and you get a clean before-and-after once you've made changes. Inherent availability then shows the ceiling those two levers put on production, before planning and logistics delays get added on top.

Common questions

How do you calculate MTBF?

Divide total operating time by the number of failures in that time. An asset that ran 8,000 hours and failed 12 times has an MTBF of about 667 hours. Use operating hours rather than calendar hours, and count only real failures, not planned work.

How do you calculate MTTR?

Divide the total time spent restoring the asset by the number of repairs. Twelve repairs taking 48 hours in total give an MTTR of 4 hours. Decide whether the clock includes diagnosis and waiting for parts, and apply that rule every time.

What is the difference between MTBF and MTTF?

MTBF applies to repairable assets that go back into service after a failure. MTTF, mean time to failure, applies to items that are replaced rather than repaired, such as bearings or seals.

What is a good MTBF?

There is no universal figure, because MTBF depends on the equipment, its duty and how failures are counted. Compare an asset against its own history and against similar equipment on the same site, and watch the trend after maintenance changes.

How does MTBF relate to availability?

Inherent availability is MTBF divided by the sum of MTBF and MTTR. Improving either raises availability. Fewer failures lift MTBF, and faster repairs cut MTTR.

Want to improve the numbers?

These calculators reflect the work we do every day. If you want help lifting availability, cutting downtime or tightening your schedule, let's talk.