Often you will see that a web hosting company offers, for example, “99.5% guaranteed uptime”. What does this mean, and why is it important?

Put simply, uptime is the amount of time that your web site is up and running, and available to the world. In contrast, downtime means that your web site will be inaccessible. Uptime is normally given as a percentage, as above. Let’s do some math, and see exactly what is meant by 99.5% uptime. A calendar month contains 43,200 minutes, so an uptime figure of 99.5% means that your site should be down for a maximum of 216 minutes during any one month. If the uptime figure was 99.7%, then the downtime drops to 130 minutes a month, or a little over two hours.

That all sounds very impressive, however it’s not quite as simple as that. An uptime guarantee is not a promise that your web site will be available for that amount of time. Rather, it is the target uptime that the web hosting company is aiming for, normally with some form of compensation offered for those times that the company fails to meet that target.

The compensation offered may take a couple of different forms. One standard method is to reduce the users’ fees by the amount of downtime, so for example if the uptime was actually 95% then the fees would be reduced by 5%. Another way is to offer a discount on the fees paid, again with the amount being determined by the actual downtime. Bear in mind though that the user will need to know that his site is down for that amount of time, as any compensation will need to be claimed – the web hosting company will not offer it unasked. Remember too that the compensation is actually quite limited compared to what any lost sales may have been worth.

One thing that needs to be checked when considering an uptime guarantee is any clause in the Terms of Service that may limit the definition of that guarantee. Nearly all web hosting companies will exclude natural disasters, such as hurricanes or earthquakes, which is fair enough. However, some companies will also exclude hardware or software upgrades from the guarantee, which can leave that company a loophole. Of course there are certain times when a web site may be inaccessible for genuine reasons – servers do need to be maintained or upgraded, and software needs regular patches, normally requiring a re-boot when done. However, if a company is claiming to do this more often than you think necessary, they may be using that loophole.

An uptime guarantee is a nice thing to have from a web hosting company, especially if backed by some form of compensation. However please remember that it is just a target figure and may not be reached each and every month. Don’t be swayed by large advertisements promising 99.99% uptime – much better to check what that company’s actual uptime has been lately by contacting existing clients.