How it works
MTBF is total operating time divided by the number of failures. MTTR is total repair time divided by the number of repairs. Together they give inherent availability, the share of time the equipment is able to run when only failures are counted.
The probability of running a given period without failure assumes a constant failure rate (the exponential model). That suits random failures in electronics and mature equipment; it doesn't capture wear-out, which predictive maintenance targets.
MTBF = operating time / failures
MTTR = total repair time / repairs
Availability = MTBF / (MTBF + MTTR)
λ = 1 / MTBF R(t) = e^(−t / MTBF)Worked example
A pump runs 2,000 hours with 4 failures and 10 hours of total repair time. MTBF = 500 h, MTTR = 2.5 h and availability = 500 / 502.5 = 99.50%. The chance of running a 100-hour week without failure is e^(−100/500) = 81.9%.