MTBF & Availability
MTBF, MTTR, failure rate, inherent availability, mission reliability, and the MTTR or MTBF a target availability would need — from a maintenance log.
Between failures,
and back to work.
What MTBF measures, what it assumes, and what a small count can and cannot say.
The definitions
MTBF = operating time ÷ failures; MTTR = total repair time ÷ failures; failure rate λ = 1 ÷ MTBF, also shown per million hours. Inherent availability = MTBF ÷ (MTBF + MTTR) — the share of time the equipment is up when only corrective repairs count. Downtime per year is (1 − A) × 8,760 h. Operating time means time the equipment was actually running or required to run, not calendar time.
Mission reliability and targets
R(t) = e^(−t ÷ MTBF) is the probability of running a mission of length t without a failure, assuming a constant failure rate. A mission as long as the MTBF survives only 37 % of the time — MTBF is an average, not a guarantee. Given a target availability, the utility also solves for the MTTR you would need at the current MTBF, and the MTBF you would need at the current MTTR, so you can see which lever is closer.
What this does not cover
The exponential model ignores wear-out and infant mortality (Weibull analysis handles those), and a handful of failures gives a rough MTBF — the page warns below five. Zero failures cannot give an MTBF, only a lower bound. Planned maintenance, logistics delay, and spares are outside inherent availability (that would be operational availability). Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.