Work out reliability at time t instantly with clear inputs, formula shown and shareable results.
For a constant failure rate, reliability decays exponentially: R(t) = e^(-t/MTBF). Failure rate lambda is the reciprocal of MTBF. Multiplying the complement of reliability by the fleet size gives the expected number of failures in the mission, and MTBF against MTTR gives inherent availability.
Exponential reliability
R(t) = e^(-t / MTBF); failure rate = 1 / MTBF
Availability
Inherent availability = MTBF / (MTBF + MTTR)
During the useful-life phase where failures are random. It does not describe infant mortality or wear-out, which need a Weibull model with shape parameter below or above 1.
Because e^-1 = 0.368. MTBF is the mean, not a guarantee period; most items fail before reaching it under an exponential distribution.