The standard measure of reliability for hosting and network providers is a Service Level Agreement, or SLA, measured as a percentage of uptime. But do you really understand what that percentage means? Does that uptime really meet your business needs?
The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously. Service level agreements (SLAs) often refer to monthly downtime in order to calculate service credits to match monthly billing cycles.
| Availability % | Downtime per year | Downtime per month* | Downtime per week |
|---|---|---|---|
| 90% | 36.5 days | 72 hours | 16.8 hours |
| 95% | 18.25 days | 36 hours | 8.4 hours |
| 98% | 7.30 days | 14.4 hours | 3.36 hours |
| 99% | 3.65 days | 7.20 hours | 1.68 hours |
| 99.5% | 1.83 days | 3.60 hours | 50.4 min |
| 99.8% | 17.52 hours | 86.23 min | 20.16 min |
| 99.9% (“three nines”) | 8.76 hours | 43.2 min | 10.1 min |
| 99.95% | 4.38 hours | 21.56 min | 5.04 min |
| 99.99% (“four nines”) | 52.6 min | 4.32 min | 1.01 min |
| 99.999% (“five nines”) | 5.26 min | 25.9 s | 6.05 s |
| 99.9999% (“six nines”) | 31.5 s | 2.59 s | 0.605 s |
* For monthly calculations, a 30-day month is used.
It should be noted that uptime and availability are not synonymous. A system can be up, but not available, as in the case of a network outage. The hosting server might remain up, but if the network connected to it goes offline, will you qualify for a service credit for the outage?
