Availability (nines) calculator
Availability calculator. Stack components in serial or parallel; see combined nines and downtime.
About this tool
Stack components in series or in parallel to see the combined availability, how many nines that is, and the downtime it allows per day, week, month, and year. The URL holds the stack, so you can share it.
Good for
- Showing why five 99.9% dependencies in series won't give you 99.9%
- Checking what a redundant pair actually buys you
- Setting an SLO you can meet given what you depend on
Not for
- Correlated failures: the parallel maths assumes components fail independently, and a shared DNS provider, region, or deploy pipeline breaks that assumption
- Predicting incidents: availability figures are averages, not schedules