Uptime and SLA calculator
What an availability promise actually allows: “99.9 %” in minutes per month, how much a given outage costs in percentage terms, and what happens when services depend on each other or back each other up.
From downtime to availability
Combining components
A service that needs every component working is less available than any one of them. One with redundant copies is more available than any one of them — if they fail independently.
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What each extra nine costs
- 99 % — about 7 hours a month. A single server with ordinary care.
- 99.9 % — about 44 minutes a month. Typical for a managed service.
- 99.99 % — about 4 minutes a month. Needs redundancy and automatic failover.
- 99.999 % — about 26 seconds a month. Needs redundancy everywhere, including the people.
How long is a month here?
A twelfth of a year, 30.44 days — the convention most published SLA tables use. A contract may define it as a calendar month instead, which changes the figure slightly.
Does planned maintenance count?
That depends entirely on the contract. Many SLAs exclude announced maintenance windows, which is why the percentage alone does not tell you how often a service may be down.
Why does adding a component lower availability?
If a request needs a web server, a database and a network link all working, each one’s downtime adds to the total. Three 99.9 % components in series give about 99.7 %.
Is anything I type sent anywhere?
No. The calculation runs in your browser.