What is uptime?
Published
Uptime is the percentage of time a system is available and usable over a given period (monthly or annually). Its complement is downtime - and the difference between a near-100% figure and one lower by a small decimal can be the difference between hours and days of outage.
The numbers translate into time
A table worth memorising - the annual downtime allowed at each tier:
| Percentage | Annual downtime | Monthly downtime |
|---|---|---|
| 99% | 3.65 days | 7.2 hours |
| 99.5% | 1.83 days | 3.6 hours |
| 99.9% ("three nines") | 8.77 hours | 43.2 minutes |
| 99.95% | 4.38 hours | 21.6 minutes |
| 99.99% ("four nines") | 52.6 minutes | 4.3 minutes |
| 99.999% ("five nines") | 5.26 minutes | 26 seconds |
Interpretation: a system promising 99% "almost always available" - allows itself three and a half days of outage a year. The gap between 99% and 99.9% is not modest; it is the difference between "may be down for days" and "may be down for hours" - roughly tenfold.
Promise or commitment?
Uptime on the marketing page is a marketing number. A Service Level Agreement (SLA) is a contractual commitment with compensation on breach (invoice credits, free months, ...).
A serious provider publishes a clear SLA with its terms: what counts as downtime, what is excluded (scheduled maintenance with adequate notice, force majeure), and how to claim compensation. A provider stating "99.9%" on the website without an SLA promises without committing.
What this means for your business
- An accounting system at 99.9%: potential 43 minutes of monthly outage - acceptable for businesses not invoicing by the second.
- A closer requirement of 99.95%-99.99%: for critical e-invoicing, a retail till dependent on connectivity, or industries with time-critical peaks (month-end for filing).
Two questions for a provider before buying:
- What is the uptime SLA, and where is its text?
- Does a public status page transparently show actual outage history?
A public status page is a quick test of the provider's transparency culture - the best cloud providers publish every incident openly with post-mortems.
Frequently asked questions
Does scheduled maintenance count as downtime?
Under most SLAs it doesn't, if announced with adequate notice and falling in a stated maintenance window. Read the terms.
My internet went down - is that "system downtime"?
No - the cloud system is up; you can't reach it. The problem is at your end or your ISP's. Downtime counts from the provider's servers, not from the user.
How do I prove the system was down?
The public status page usually reflects incidents with their timing. An SLA claim usually requires notice of the event within a defined window.