What does 99% uptime mean?
99% allows 3d 15h 36m of downtime a year — 7h 12m a month, or 1h 40m 48s a week.
| Per day | 14m 24s |
|---|---|
| Per week | 1h 40m 48s |
| Per month (30 days) | 7h 12m |
| Per year (365 days) | 3d 15h 36m |
An availability target is a downtime budget, and it is easier to judge as time than as a percentage. At 99%, a service may be unavailable for 7h 12m in a month and still have met the target.
Whether that is generous or tight depends on what the target excludes. Most SLAs do not count maintenance announced in advance, and many exclude faults in networks the provider does not run — so two providers quoting the same number can be promising quite different things.
One step tighter, 99.5%, cuts the yearly budget to 1d 19h 48m. One step looser, 98%, raises it to 7d 7h 12m.
How it is measured matters as much as the number. A check every five minutes cannot see a two-minute outage, and a single probe cannot tell an outage apart from its own bad network.
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