Allowed downtime for an availability target
Availability from an outage
Combined availability of several components
What the nines mean
An availability target is the share of time a service must be working. Each extra “nine” cuts the allowed downtime by a factor of ten:
| Availability | Per month | Per year |
|---|---|---|
| 99% (two nines) | 7h 18m | 3d 15h 39m |
| 99.9% (three nines) | 43m 50s | 8h 45m 58s |
| 99.95% | 21m 55s | 4h 22m 59s |
| 99.99% (four nines) | 4m 23s | 52m 36s |
| 99.999% (five nines) | 26s | 5m 16s |
The calculator uses calendar averages: a year of 365.25 days, a month of 30.44 days (a twelfth of a year) and a quarter of three such months. A contract may define the month as the actual calendar month, which changes the figures slightly; it may also exclude scheduled maintenance.
Turning an outage into a percentage
Enter how long a service was down and the period it is measured over. A number on its own is read as minutes. This is how an SLA credit is usually worked out: compare the result with the promised percentage.
Availability of a whole system
When a request has to pass through several components, such as a load balancer, an application server and a database, all of them must be up, so their availabilities multiply: three components at 99.9% give about 99.7%. A chain is always less available than its weakest link.
Redundancy works the other way. With two independent servers where either one is enough, the system is down only when both are, so the chances of failure multiply: two servers at 99% give 99.99%. This assumes failures are independent; servers sharing a power feed, a network switch or a bad deploy fail together, so real-world gains are smaller.
An error budget is the downtime your target leaves you, often spent on deployments and maintenance. All calculations run in your browser.