MTBF and MTTR Calculator: Formula, Example & Instant Result
MTBF (mean time between failures) is the average operating time between failures of a repairable item; MTTR (mean time to repair) is the average time it takes to get it running again. Together they give the inherent availability: the share of time the equipment is able to run.
β³ Values are pre-filled with an example β replace them with your own data.
Your data
h
Unplanned stops that needed a repair, counted by one fixed rule.
h
Result
MTBF120.0 h
On average the equipment runs 120.0 hours between failures.
MTTR1.58 h
Average time from failure to running again.
Availability98.70%
Share of time the equipment was able to run.
Why the two availability formulas agree
T / (T + R)
= (T/F) / (T/F + R/F) divide top and bottom by F
= MTBF / (MTBF + MTTR)
MTBF is the average running time between failures over this period β not a lifetime, service life or warranty.
Observation period as 6 cycles of about 120.0 h running, each followed by about 1.58 h of repair (short repairs are drawn wider so they stay visible).
Formula
MTBF = total operating time / number of failures
MTTR = total repair time / number of failures
Availability = operating time / (operating time + repair time)
= MTBF / (MTBF + MTTR)
Worked example
A packaging robot ran 720 hours last month and failed six times; repairs took 9.5 hours in total. How reliable was it?
Data
β’ Operating time T = 720 h
β’ Failures F = 6
β’ Total repair time R = 9.5 h
Step by step
1MTBF = 720 / 6 = 120 h
2MTTR = 9.5 / 6 = 1.58 h
3Availability = 720 / (720 + 9.5) = 98.7 %
4Check: 120 / (120 + 1.58) = 98.7 % β
What it means
On average the robot runs 120 hours between failures and is back up in about 1.6 hours, giving 98.7 % availability. That does not mean it will last 120 hours or that it wears out after 120 hours β it is an average over this period. With six failures, look at whether they share a cause before deciding between faster repair and fewer failures.
Tips & common mistakes
β²Count only operating time, not calendar time; idle and planned shutdowns are not exposure.
β²Define what counts as a failure (any unplanned stop? only those needing repair?) and keep it fixed.
β²Include waiting for technicians and parts in repair time if you want to see real availability.
β²Report the observation period with the result: 120 h MTBF from 6 failures is weaker evidence than from 60.
β²AI assistants often treat MTBF as expected lifetime; check that their interpretation matches the definition here.
AI prompt for this calculation
Paste it into your AI assistant to be walked through the calculation with your own data.
Act as a reliability engineer and help me calculate MTBF, MTTR and availability for a repairable machine.
Use exactly these formulas:
MTBF = total operating time / number of failures
MTTR = total repair time / number of failures
Availability = operating time / (operating time + repair time) β show that this equals MTBF / (MTBF + MTTR).
Rules:
1. Work step by step, in hours.
2. Restate my numbers first and check the decimal separators.
3. If operating time, failure count or repair time is missing or unclear, ask me β never assume.
4. If there were no failures, say MTBF is undefined and report the observation time instead; never divide by zero.
5. Remind me that MTBF is not a service life or warranty period, and suggest whether to focus on fewer failures or faster repair.
My data:
Operating time (h) =
Number of failures =
Total repair time (h) =
What counts as a failure =
The buttons copy the prompt first, then open the assistant in a new tab.
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Next step Β· DOCAMIN
Build a reliability report and maintenance plan from these numbers in DOCAMIN.
Is MTBF the same as service life or warranty period?
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No β this is the most common misreading. MTBF is the average operating time between failures in the period you observed, the inverse of the failure rate. An item with an MTBF of 120 hours does not last 120 hours, and a component with an MTBF of 100,000 hours does not last eleven years; it means that across many units, failures occur at that average rate.
What counts as one failure?
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Decide it in writing before you count: usually any unplanned loss of function that needs intervention. Repeated stops from the same unresolved fault may count as one failure or several β choose one rule and apply it every time, or the MTBF will move for reasons that have nothing to do with the equipment.
How does scheduled maintenance affect MTBF?
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Planned maintenance is not a failure and its time is not operating time, so leave it out of both. Good preventive maintenance should show up as a rising MTBF over time; if it doesn't, the maintenance tasks may not target the real failure modes.
When do simple averages mislead?
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When the failure rate changes over time: early-life failures, wear-out at the end of life, or a few long repairs that dominate MTTR. Then a single average hides the pattern, and a reliability analysis that fits a failure-time distribution gives a truer picture.
Last tested: October 2026 Β· Tested with: automated math tests (3+ cases) Β· Reviewed by: Quality for Everyone editorial team
