MTBF, MTTR & availability calculator
Turn one set of observed operating, failure and active corrective-repair totals into MTBF, MTTR, failure rate and a clearly bounded availability estimate.
Total time the asset was operating inside the observation boundary.
Whole number of failures recorded against that operating time; at least one is required.
Total active corrective-repair time for those failures. Zero is allowed.
Entered operating time divided by operating plus active corrective-repair time.
- Observed MTBF
- 200 hr
- MTTR
- 10 hr
- Observed failure rate
- 0.005 per hr
- Modeled observation window
- 1,050 hr
- Active-repair share
- 4.76%
Entered-time composition
This is descriptive arithmetic from the entered totals. It does not establish constant failure or repair rates, predict future reliability or include downtime outside active corrective repair.
Included
- Observed operating time per recorded failure (MTBF in this calculator)
- Mean active corrective-repair time per recorded failure (MTTR)
- Observed failure rate per selected time unit
- An inherent-availability estimate from the entered operating and active repair totals
Not included
- A forecast of future failures, reliability probability or maintenance interval
- Logistics delay, administration, preventive maintenance and other downtime
- Achieved or operational availability
- Failure distributions, confidence intervals or a design lower bound
What this means
In this calculator, observed MTBF is aggregate operating time divided by the number of recorded failures. Definitions of “between failures” vary in practice, so the result is labelled as operating time per observed failure rather than treated as a universal interval convention.
MTTR is the entered active corrective-repair time divided by the same failure count. It deliberately excludes logistics, waiting, administration, preventive maintenance and any downtime not represented by that input.
The availability figure reconciles the entered totals in two equivalent ways: operating time ÷ (operating time + active repair time), and MTBF ÷ (MTBF + MTTR). It is a descriptive estimate under this narrow inherent-availability definition—not achieved availability, a probability of failure-free operation or a forecast.
Formula & worked example
observed MTBF = aggregate operating time ÷ observed failures
MTTR = aggregate active corrective-repair time ÷ observed failures
failure rate = observed failures ÷ aggregate operating time
estimated inherent availability = MTBF ÷ (MTBF + MTTR)
= operating time ÷ (operating time + active corrective-repair time)1,000 operating hours, 5 observed failures and 50 hours of active corrective repair
- Observed MTBF: 1,000 ÷ 5
- 200 hr
- MTTR: 50 ÷ 5
- 10 hr
- Entered-time availability: 1,000 ÷ (1,000 + 50)
- 95.24%
The entered totals describe 200 operating hours per observed failure, 10 active repair hours per failure and an estimated inherent availability of 95.24% within the modeled 1,050-hour window.
How this calculation works
The calculation follows the formulas, definitions and assumptions explained on this page. The references below support the method and any stated boundaries.
Official sources
- NASA — Inherent Availability and Reliability with Constant Failure and Repair Rates — Conventional relationship between MTBF, MTTR and inherent availability
- NASA/Kennedy Space Center — Availability: What is it? — Distinctions between inherent, achieved and operational availability
- VDMA — 5G integration guide — German industrial use of MTBF, MTTR and the availability relationship