What does 98% uptime mean?
98% allows 7d 7h 12m of downtime a year — 14h 24m a month, or 3h 21m 36s a week.
| Per day | 28m 48s |
|---|---|
| Per week | 3h 21m 36s |
| Per month (30 days) | 14h 24m |
| Per year (365 days) | 7d 7h 12m |
An availability target is a downtime budget, and it is easier to judge as time than as a percentage. At 98%, a service may be unavailable for 14h 24m 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%, cuts the yearly budget to 3d 15h 36m. One step looser, 95%, raises it to 18d 6h.
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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