Free tool
MTBF and availability calculator.
Enter the hours an asset was scheduled to run, the hours it was down, and how many times it failed. You get mean time between failures, mean time to repair, availability and failure rate.
Downtime is greater than the scheduled hours. Check the period, an asset cannot be down for longer than it was meant to run.
The period, hour by hour
The formulas
Operating time is what the asset actually ran, so downtime comes out first:
operating time = scheduled hours - downtime hours
MTBF = operating time / number of failures
MTTR = total downtime / number of failures
availability = MTBF / (MTBF + MTTR)
That last one is the same as dividing operating time by scheduled time, which is why the availability on this page matches both routes. If you have ever seen two reports disagree on uptime, this is usually where it started.
The mistake that inflates MTBF
Plenty of teams divide the whole calendar period by the failure count and call it MTBF. That folds repair hours into running hours and quietly makes the asset look more reliable than it is. MTBF is about time between failures, so the hours it spent in the workshop do not belong in the numerator.
MTBF and MTTR pull different levers
MTBF is a reliability number. It moves when you change the machine, the operating conditions, or the preventive maintenance regime. MTTR is a maintenance response number. It moves when you change parts availability, crew scheduling, and how quickly a fault gets diagnosed.
Two assets with identical availability can need completely different fixes. One fails rarely and takes three days to repair. The other fails weekly and is back in an hour. Averaging them into a single uptime figure hides which problem you actually have.
A note on repairable assets
MTBF is for assets you repair and put back into service: straddle carriers, haul trucks, cranes, conveyors, forklifts. For parts you replace rather than repair, the equivalent measure is MTTF, mean time to failure. The arithmetic looks similar, the meaning does not.
The number depends on the clock starting on time
Every figure here rests on two inputs: when the asset went down, and when it came back. In practice that clock starts when someone calls it in over the radio and stops when a mechanic mentions it is running again. If the work order gets written up at the end of the shift, the recorded downtime is whatever somebody remembered.
That is why MTTR so often looks better on paper than the yard feels. The repair time is measured from the paperwork, not from the moment the machine stopped.
