- Home
- Calculators
- MTBF and MTTR Calculator
MTBF and MTTR Calculator: Reliability and Availability
MTBF, MTTR and the availability they set.
Fewer than 10 failures: treat MTBF as a rough estimate.
The period's hours, failure by failure
Do this To reach 97.5% availability: 17.4 h of repair a period at most, down from 20.0 h. That is MTTR 2.17 h at 8 failures, or 6.9 failures a period at 2.50 h.
The MTBF and MTTR formulas
Inherent availability = MTBF ÷ (MTBF + MTTR). Failure rate λ = 1 ÷ MTBF.
How to use this calculator
- Enter operating hours time running or available to run in the period, excluding repair time.
- Enter the number of failures unplanned breakdowns that needed repair, by your written definition.
- Enter total repair hours breakdown to back in service, summed over all failures.
What MTBF and MTTR tell you
MTBF (mean time between failures) is how long equipment runs, on average, before it breaks down. It is the reliability number: preventive maintenance, operator care and design fixes raise it. MTTR (mean time to repair) is how long it takes, on average, to get it running again. It is the maintainability number: spare parts on hand, trained technicians, standard repair procedures and quick diagnosis lower it.
Together they set inherent availability, the share of time the equipment could run if breakdowns were the only loss.
Worked example
A CNC machine logged 680 operating hours last month, 8 breakdowns, and 20 hours of repair in total. These are the numbers loaded in the calculator above.
MTTR = 20 ÷ 8 = 2.5 hours
Inherent availability = 85 ÷ (85 + 2.5) = 85 ÷ 87.5 = 97.1%
Failure rate = 1 ÷ 85 = 0.0118 per hour = 11.8 per 1,000 hours
Two ways to gain: halving MTTR to 1.25 hours takes availability to 85 ÷ 86.25 = 98.6%. Doubling MTBF to 170 hours gives 170 ÷ 172.5 = 98.6% as well, and also halves the number of disruptions to the schedule. The second is usually harder but worth more.
Getting the inputs right
- Operating time excludes repair. If you only have calendar or scheduled hours, subtract the repair hours first, or MTBF comes out high.
- One failure definition. Write down what counts (for example, any unplanned stop needing maintenance, or over 10 minutes). Changing the rule changes MTBF more than any improvement will.
- Split MTTR when it is long. Time to respond, time to diagnose, time waiting for parts and time to repair point at different fixes. A 2.5 hour MTTR with 1.5 hours waiting for parts is a stores problem, not a skills problem.
- Use enough failures. MTBF from 2 failures is a guess. Roll up over a quarter, or across identical machines, until there are 10 or more.
For the full loss picture including changeovers and slow running, use the OEE calculator. To put a cost on the repair hours, use the downtime cost calculator. The TPM guide and preventive and predictive maintenance guide cover how to raise MTBF.
Get the Plant Floor Briefing: one free tool like this every 2 weeks
One guide, one free tool, one problem fixed. Every 2 weeks by email. No spam, unsubscribe any time.
Frequently asked questions
How do you calculate MTBF?
MTBF = operating time / number of failures. Operating time is the time the equipment ran or was available to run, not counting the time it was down for repair. 680 operating hours with 8 failures gives an MTBF of 85 hours.
How do you calculate MTTR?
MTTR = total repair time / number of failures. Repair time runs from the breakdown to the equipment being back in service, including waiting for a technician or parts unless you track those separately. 20 repair hours over 8 failures gives an MTTR of 2.5 hours.
What is the difference between MTBF and MTTF?
MTBF is for repairable equipment that is fixed and put back in service. MTTF (mean time to failure) is for items that are replaced, not repaired, such as bearings, lamps or fuses. The arithmetic is the same: operating time over failures.
How is availability from MTBF and MTTR different from OEE availability?
Inherent availability = MTBF / (MTBF + MTTR) counts only breakdowns and their repair. OEE availability = run time / planned production time and also counts changeovers, material waits and other stops. Inherent availability is the ceiling the maintenance side sets; OEE availability is what production actually got.
What counts as a failure?
Decide once and write it down. A common rule is any unplanned stop that needs a maintenance intervention, or any stop longer than a set threshold such as 5 or 10 minutes. Shorter stops cleared by the operator are usually minor stops, counted in OEE performance, not as failures.
Dive deeper
Free Process Modeler. Map a full production flow, find the bottleneck, and plan staffing. Free in the browser.
Open the modeler