Uptime is the amount of time that a server has stayed up and running. A guarantee is a commitment that a company makes to its customers. An uptime guarantee is the guarantee promising uptime or life of the server that is agreed upon.
In other words, it is a guarantee that the server your website is on will be online and available a certain percentage of time.
What the Guarantee Does Not Include
Many hosts schedule maintenance at least once a month to install software updates, check the integrity of the hard drive and perform any fixes and optimizations, replace or upgrade hardware, etc.
So, for example, if your internet service provider (ISP) has a link down and you can't reach your website, your web host is not responsible, since the server is still up and available to other customers on other ISPs.
Getting a Credit for Downtime
If your server is down for an extended period of time, you may be eligible for a credit to your account. Reputable web hosts will give you a credit for any downtime when downtime has exceeded their guarantee by their measurements. They typically check the server logs and see how long the server was down on their end to calculate the downtime.
Most hosts will not give you a credit unless you ask, unless it is a highly publicized or widespread outage. Agents usually have some discretion, so it is recommended that you be polite when asking for a credit. (Rude or hostile customers may find they "do not quality" based on the "server logs" not showing enough downtime. It's always better to be polite.)
Web hosts will typically not accept your record of downtime, and instead will rely on their own internal server logs and records.
Downtime Allowed Under Guarantee
The amount of downtime allowed under the guarantee depends on the exact percentage they specify in their agreement.
- 99% = 7.2 hours in a 30 day period.
- 99.9% = 1 hour in a 30 day period (technically 0.999 hours or 59.94 minutes)
- 99.99% = 4.32 minutes in a 30 day period
Keep in mind that the allowed downtime typically does not include scheduled maintenance. So if the server is down for an hour while they check the disk's integrity, that does not count as downtime under the guarantee.
See your web host's guarantee for specifics terms and conditions.