Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the MTBF, MTTR & availability calculator
Summarize repairable-equipment failure and repair records for a consistent reporting period. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Total operating time (hours). Total time the equipment actually operated, in hours, excluding downtime.
- Number of failures. Number of failures in the same period, as a whole number of at least 1. The minimum is 1.
- Total repair time (hours). Total repair time for those failures, in hours.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
Use a consistent observation period and a clear definition of failure. Operating time excludes downtime, while repair time should cover the failures counted in that same dataset. Inherent availability is a simplified model, so planned maintenance, logistics delay and waiting for spare parts require separate treatment when reporting operational availability.
MTBF = operating time / failures. MTTR = repair time / failures. Availability = MTBF / (MTBF + MTTR) × 100%.
Operating time excludes downtime. Count failures and their repair time over the same period. This simplified inherent availability excludes planned downtime and other delays. With no failures, an observed MTBF cannot be estimated by this method.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 96.7742 % availability. MTBF: 300 h · MTTR: 10 h
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These illustrative rows retain the worked example assumptions except for the varied input. They are not certified ratings or manufacturer data. Use the form for intermediate values.
Common mistakes
Using calendar hours as operating hours; counting minor stops differently between periods; treating a no-failure period as proof of infinite reliability. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
The results are the mean time between failures, the mean time to repair and the inherent availability for your data. They describe the period you entered and are not a prediction.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. Operating time excludes downtime. Count failures and their repair time over the same period. This simplified inherent availability excludes planned downtime and other delays. With no failures, an observed MTBF cannot be estimated by this method. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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