Hosts advertise uptime as a percentage. The numbers look almost the same, but the downtime they allow is very different.
| Uptime | Downtime per month | Downtime per year |
|---|---|---|
| 99% | about 7 hours 18 minutes | about 3 days 15 hours |
| 99.5% | about 3 hours 39 minutes | about 1 day 20 hours |
| 99.9% | about 44 minutes | about 8 hours 46 minutes |
| 99.95% | about 22 minutes | about 4 hours 23 minutes |
| 99.99% | about 4 minutes 23 seconds | about 52 minutes |
Months here are the average month, a twelfth of a year.
What an uptime guarantee usually promises
An uptime guarantee, part of a service level agreement (SLA), is a promise of compensation, not of uptime. If the host misses the target, you typically get a credit off next month’s bill, often a small percentage, and usually only if you claim it.
Read the details:
- What counts as down? Often only the host’s network or hardware. Your site crashing because of its own software does not count.
- Planned maintenance is usually excluded.
- How is it measured? The host’s own monitoring, from their own network, is common.
Why it still matters
The figure tells you how a host designs its systems. Reaching 99.99% needs redundant power, network paths and hardware. A host promising 99% is telling you outages of several hours are possible.
Measuring your own uptime
Use an external monitor that checks your site every minute or so from outside your host’s network and alerts you when it fails. Many free services do this. The record they keep is far more useful than a percentage on a sales page.
Something out of date? Software changes. If a step no longer works, tell us and we will check it and update the page.
