Formula
Allowed downtime = (1 − availability) × period. A month here is the average month (30.44 days).
Examples
- 99% (“two nines”): 7 h 18 min per month
- 99.9% (“three nines”): 43 min 50 s per month, about 8 h 46 min per year
- 99.99%: 4 min 23 s per month
- 99.999%: 26 s per month
Services in series
If your site depends on several services and any one of them can take it down, multiply the availabilities: three services at 99.9% give about 99.7%.
Frequently asked questions
Does planned maintenance count as downtime?
It depends on the contract. Many SLAs exclude announced maintenance windows.
How do I get more nines?
Redundancy: several servers, regions or providers in parallel, so one failure does not stop the service.
See also
Results are estimates for information only and are not financial, legal or medical advice. Rates, taxes and fees vary — confirm with a professional or your lender.