The short answer is no. While an uptime guarantee doesn't hurt, it also doesn't mean you'll have reliable web hosting service. To explain why, we'll start from the beginning.
Your web site is stored on servers at the web hosting company. These servers are computers, and just like your home PC, they can lock up, crash, or otherwise malfunction. Obviously, you would like your web site available 100% of the time, but with computers this is impossible. You'll need to settle for a reliable web host that knows how to keep their computers running well, so your web site will be available a very high percentage of the time.
Uptime is the amount of time a server and/or network is up and running correctly. It is typically given in a percentage—the percentage of time it has been up. For example, if a server was down (not working) for a total of 2% of the time last month (around 14 hours), the uptime for the month would be 98% (100 - 2).
Many web hosts have an uptime guarantee—a guarantee that your site will be available a high percentage of the time. One may offer a 99% guarantee, another 99.5% or 99.9%.
You may think that it is best to go with whoever has the highest number. However, in reality, these numbers have little to do with the actual reliability of the web host. If you read the fine print, you will likely find that you do not get much if they do not meet their uptime goals. Therefore, an uptime guarantee usually doesn't mean much.
Computers are not perfect, so with every host you will have some downtime. However, if you experience frequent or lengthy downtime for your site, or if customer service is non-responsive to problems, it is time to look for a new web host.
If you are now ready to choose a web host, go to our Web Hosting Comparison to find a web host that fits your needs. Otherwise, browse the other sections of our web hosting FAQ to learn more about hosting:
If you want to learn more of the basics about hosting in addition to what was covered in our web hosting FAQ, please see our guide on how to Choose a Web Host.