Convert MTBF and MTTR into availability, the nines it earns and the downtime allowed per year, month and week.
Availability is the share of time the system is up, which is repair-time-weighted rather than failure-count-weighted. Truly independent redundant paths multiply the unavailabilities, which is why two 99.9% paths approach 99.9999% — provided they share no fibre, power feed or software version. SLA credits are written in nines but paid in minutes, and the per-month figure is the one an operations team can actually manage against.
SLA Availability
availability = MTBF ÷ (MTBF + MTTR); for N independent parallel paths, unavailability is raised to the power N.
Downtime budget
Downtime per year = (1 − availability) × 8760 hours, so 99.9% allows 8.77 hours.
availability = MTBF ÷ (MTBF + MTTR); for N independent parallel paths, unavailability is raised to the power N. Availability is the share of time the system is up, which is repair-time-weighted rather than failure-count-weighted. Truly independent redundant paths multiply the unavailabilities, which is why two 99.9% paths approach 99.9999% — provided they share no fibre, power feed or software version.
SLA credits are written in nines but paid in minutes, and the per-month figure is the one an operations team can actually manage against.
This calculator takes 3 inputs: Mean time between failures, Mean time to repair, Independent redundant paths. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.