Tools
Uptime and SLA calculator
A service is only as available as the chain of things it depends on. Add the parts of your setup, mark which ones are duplicated, and see where the hours of downtime come from.
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Result
How the uptime and SLA calculator works
Downtime = (1 - availability) × period. A year is 365 days (8,760 hours), a month is one twelfth of that.
Components in a chain multiply: A = A1 × A2 × … × An. A component duplicated n times, where any one copy is enough, has availability 1 - (1 - A)^n.
The model assumes failures are independent and failover is instant. Shared power, shared cabling and untested failover break both assumptions, so treat the result as the best case.
Planning estimates. They do not replace a measurement on your own systems.
Terms used here: AvailabilityClusterHypervisorSingle point of failureSLA
In practice
99.9% sounds high, but it allows 8 hours 46 minutes of downtime a year. 99.99% allows 53 minutes, and 99.999% about 5 minutes.
In a chain, availabilities multiply, so six components at 99.9% each give about 99.4%, or 52 hours of downtime a year. Duplicating the weakest component usually gains more than improving the strongest one.
Questions about this tool
Is a 99.9% SLA good enough?
For most internal business systems, yes. For payment or online gaming platforms, where an hour of downtime has a direct revenue cost, 99.95% or better is common.
Why doesn't the model match our real downtime?
It assumes independent failures and instant failover. Shared power, shared cabling, maintenance windows and untested failover add downtime in practice. Treat the result as the best case.
Does planned maintenance count?
That depends on the SLA. Many contracts exclude announced maintenance windows, so read the definitions before comparing providers.
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