To have a website on the Internet, the most basic needs you require are disk space and bandwidth.
Disk space is the amount of data you can store on the web server. Obviously, the amount of space needed depends on the size of your website. Most websites are composed of HTML (text), images, Flash, or a combination of all.
Text is very economical, it occupies very little space. Images and flash are more expensive, as they require more disk space.
Ideally, your web page should stay under 50-60KB, including images and flash. This is because there are some people using slow dial-up connections of under 56Kbps. For these people, a 60KB page will take more than 8 seconds to load. 8 seconds seems like a small number, but imagine you have to sit there and stare at a blank white page for 8 seconds. That’s a lot of time. Use the slow dial-up connection as your reference. If your website loads fast using slow dial-up, it will be even faster for broadband, high-speed connection.
If your average page size is 50KB, you can put approximately 20 pages on 1Mb of space. If you have 100MB of disk space, you can host 200 pages.
Bandwidth is the amount of data you are allowed to transfer to and from your web server per month. This includes all uploads and downloads, both HTTP and FTP. Bandwidth depends on your web page size, as well as the number of visitors to your website and the number of pages they visit.
For an average page size of 50KB, 20,000 visitors per month, and 5 pages per visitor your website will need about 5000MB, or 5GB, of bandwidth per month. Note that 20,000 visitors per month equates to approximately 667 visitors per day. This is a number even most large websites cannot achieve.
If you are just launching your website, chances are you will not need a huge amount of space, nor a huge amount of bandwidth. Unless you have reasons to believe, and evidentiary numbers to support, that your website will have lots of visitors and/or you are offering movie and music downloads, there is little reason for many of us to worry about space and bandwidth. However, in choosing a web host, choose one that allows you to upgrade your disk space and bandwidth if you should need it later on.
You can use the following formula if you need to do a load test, you can determine the number of concurrent users to test if you know daily visits to your site (you may want to use Google Analytics report to find visits per hour):
Concurrent_users = (peak hourly_visits * visit_duration in seconds) / 3600