Uptime is the proportion of a time period during which a service is available and responding correctly. It is usually shown as a percentage, and the complementary figure, downtime, is the time it was not.
Why it matters
Uptime is the simplest reliability number customers and contracts understand, but small percentage differences are large in practice. The "nines" translate to allowed downtime like this (using a 30-day month and 365-day year):
| Availability | Per month | Per year |
|---|---|---|
| 99% | 7.2 hours | 3.65 days |
| 99.9% | 43.2 minutes | 8.76 hours |
| 99.99% | 4.3 minutes | 52.6 minutes |
Example
If a site was down for two incidents of 12 and 20 minutes in a 30-day month (43,200 minutes):
uptime = (43200 - 32) / 43200 = 99.926%Common mistakes
- Measuring from a single location, so a regional network problem counts as a full outage or is missed entirely.
- Counting only "server up" and ignoring a page that returns 200 with an error message. Use keyword or content checks.
- Excluding planned maintenance without stating it, which makes the number look better than users experienced.
- Comparing percentages measured with different check intervals. A five-minute check can miss shorter outages.
Related
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