MTBF, MTTR & Availability Calculator

How it works

MTBF = total operating hours ÷ failures; MTTR = total repair hours ÷ failures; availability = MTBF ÷ (MTBF + MTTR); λ = 1/MTBF; reliability R(t) = e^(−λt)

MTBF is the average operating time between failures of a repairable item, pooled across all units observed; MTTR is the average time to restore it. Their ratio gives inherent availability — the long-run share of time the item is up — which is why a component with a short MTBF can still deliver high availability if it is fixed quickly, and why redundancy and spares strategy focus on MTTR as much as on reliability. Assuming a constant failure rate (the flat part of the bathtub curve), reliability over a mission time falls exponentially, so an item with a 3,650-hour MTBF has only an 82% chance of surviving a 720-hour month; MTBF is not a lifetime. Real fleets show infant mortality and wear-out, so treat these figures as steady-state estimates and refresh them as data accumulates.

Worked example

Ten units run a year (87,600 unit-hours) with 24 failures and 96 repair hours: MTBF 3,650 h, MTTR 4 h, availability 99.89% (about 9.6 h down per unit per year). Reliability over a 720-hour month is 82%; expect about 24 failures a year across the fleet.

Frequently asked questions