MTBF and MTTR Calculator
Mean time between failures and mean time to repair from a period's failure records — with the availability they imply, the failure rate, the chance of surviving a mission time, and what halving repair time would do.
Divide operating hours by the failures and you have MTBF; divide repair hours by the failures and you have MTTR.
How the mtbf and mttr calculator works
Divide operating hours by the failures and you have MTBF; divide repair hours by the failures and you have MTTR. Availability is the share of time the equipment is up: MTBF over MTBF plus MTTR. A machine that fails monthly and takes four hours to fix is 99.5% available; one that fails weekly with the same repair is 97.6%.
Assuming failures arrive at random, the chance of running a given time without one is an exponential in that time over MTBF — the reliability figure for a shift or a mission.
Formula: MTBF = operating hours / failures; MTTR = repair hours / failures; A = MTBF / (MTBF + MTTR); R(t) = e^(−t / MTBF)
Worked examples
| Inputs | Availability | Note |
|---|---|---|
| Monthly failures, four-hour repairs | 99.455% | 99.5% available |
| Weekly failures | 97.6806% | 97.6% |
| Slow repairs | 97.3333% | MTTR of 20 hours |
FAQFrequently asked questions
What is MTBF?
Mean time between failures: operating time divided by the number of failures. It is an average of a random process, not a guarantee — a 730-hour MTBF does not mean 730 hours between every failure.
What is MTTR?
Mean time to repair, from failure to back in service — detection, diagnosis, parts and the fix. It is the number maintenance controls most directly.
What is availability?
The share of time the equipment is able to run: MTBF over MTBF plus MTTR. The two levers are failing less often and fixing faster, and the rows show each.
What does the reliability row mean?
The probability of getting through the mission time with no failure, assuming failures are random. It is the number to quote for "will it last the shift".
Does MTBF include planned maintenance?
No — it counts unplanned failures over operating time. Planned downtime is a separate term in overall equipment effectiveness.
Where these figures come from
- Nakajima (1988) — Introduction to TPM — the origin of Overall Equipment Effectiveness and its six big losses
- OSHA — the US workplace safety administration
Last checked: September 2026. Definitions follow standard operations-management practice; where plants commonly differ, the page says so.