About the MTBF & Availability Calculator
This MTBF calculator turns basic maintenance records into the core reliability metrics. Enter the total operating hours logged, the number of failures in that time and the total hours spent repairing them, and it returns mean time between failures (MTBF), mean time to repair (MTTR), failure rate, inherent availability and expected downtime per year.
It also estimates reliability — the probability a unit survives a given mission time without failing — using the exponential model that underlies MTBF. Maintenance planners, reliability engineers, plant managers and IT operations teams use these numbers to compare assets, justify spares and set realistic uptime targets.
Operating hours can be pooled across several identical units (for example 5 machines × 2,000 hours = 10,000 hours). The exponential reliability estimate assumes a constant failure rate, which fits the useful-life period of most equipment but not early “infant mortality” or end-of-life wear-out.
With the default inputs, the mtbf (mean time between failures) is 2,500 h. Change any value above to recalculate instantly.
How to use the mtbf & availability calculator
- 1Add up operating hours for the asset (or all identical assets) over the observation period.
- 2Count the failures that stopped the equipment in that time.
- 3Enter the total hours spent repairing those failures.
- 4Set a mission time, such as one shift, a week or a production run.
- 5Read MTBF, MTTR, availability and the chance of completing the mission without failure.
Formula and method
MTBF is total operating (up) time divided by the number of failures, and MTTR is total repair time divided by the number of failures. The failure rate λ is the reciprocal of MTBF. Inherent availability is the fraction of time the equipment is able to run when only corrective repair time is counted: MTBF ÷ (MTBF + MTTR). Expected yearly downtime multiplies the unavailability by 8,760 hours.
Reliability uses the exponential distribution, which assumes a constant failure rate: the chance of surviving t hours is e^(−t/MTBF). A consequence many people miss is that only about 36.8% of units survive to the MTBF without failing — MTBF is an average rate, not a guaranteed life.
- T
- Total operating hours (uptime)
- F
- Number of failures
- D
- Total repair / downtime hours
- λ
- Failure rate per hour
- t
- Mission time in hours
Worked examples
10,000 hours, 4 failures, 20 repair hours
MTBF = 10,000 ÷ 4 = 2,500 hours and MTTR = 20 ÷ 4 = 5 hours, so availability is 2,500 ÷ 2,505 = 99.80%. The chance of a 500-hour run with no failure is e^(−500/2500) = 81.9%, and about 17.5 hours of downtime are expected per year of continuous running.
One machine for a year, 2 failures
A full year of running (8,760 h) with 2 failures gives an MTBF of 4,380 hours and an MTTR of 6 hours. Availability is 99.86%, and a 1,000-hour run has a 79.6% chance of being failure-free.
Fleet of 10 units pooled, weekly mission
Ten pumps running 5,000 hours each log 50,000 hours and 10 failures: MTBF 5,000 h, or 200 failures per million hours. The probability one pump runs a 168-hour week without failure is e^(−168/5000) = 96.7%.
Frequently asked questions
How do you calculate MTBF?+
Divide the total operating time by the number of failures in that time. For example, 10,000 operating hours with 4 failures gives an MTBF of 2,500 hours. Downtime spent on repairs is not counted as operating time.
What is the difference between MTBF and MTTF?+
MTBF applies to repairable items that are fixed and returned to service; MTTF (mean time to failure) applies to non-repairable items that are replaced, such as bulbs or bearings. The calculation, total operating time divided by failures, is the same.
How is availability calculated from MTBF and MTTR?+
Inherent availability = MTBF ÷ (MTBF + MTTR). With an MTBF of 2,500 hours and MTTR of 5 hours, availability is 2,500 ÷ 2,505 = 99.8%. Operational availability is lower because it also counts waiting for parts and preventive maintenance.
Does an MTBF of 10,000 hours mean the device lasts 10,000 hours?+
No. Under a constant failure rate only about 37% of units run to the MTBF without failing, and some fail much sooner. MTBF describes the average rate of failures across a population, not the life of one unit.
What if there were no failures?+
With zero failures MTBF cannot be calculated directly; there is no point estimate. Reliability engineers instead state a minimum demonstrated MTBF from the chi-squared distribution: with zero failures the lower bound is 2T ÷ χ²(2 degrees of freedom), which works out to T ÷ 2.303 at 90% confidence (e.g. 10,000 failure-free hours demonstrates an MTBF of at least about 4,343 hours).