About this tool
Availability targets are easier to understand when converted into actual downtime. “Four nines” sounds impressive, but the operational question is how many minutes of impact can occur before the target is missed.
This calculator converts a target availability percentage and measurement period into the maximum downtime budget for that period.
How to use it
- Enter the target availability, such as 99.99.
- Enter the measurement period in days.
- Calculate and use the resulting downtime budget for SLA planning, risk discussions or maintenance analysis.
Example: 99.99% over 30 days
A 30-day period has 43,200 minutes. At 99.99% availability, only 0.01% may be unavailable, which is about 4.32 minutes.
Engineering notes
- Downtime budgets are not necessarily maintenance budgets. Many SLAs define planned maintenance separately.
- Multiple incidents consume the same monthly or annual allowance, so track cumulative qualifying downtime rather than reviewing events in isolation.
Frequently asked questions
How much downtime does 99.999% allow?
Over a 365-day year, five nines allows about 5.26 minutes, assuming every minute is in scope.
Why does the allowed time vary by month?
Because calendar months have different lengths. A percentage applied to 28 days produces a smaller downtime budget than the same percentage over 31 days.
Can I use this for internal SLOs?
Yes. The math is the same; just make sure your measurement rules are documented.