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Bandwidth is the term that refers to how much data is sent and retrieved from one source to another through the Internet - generally, from your home / work computer to the remote hosting server. The term bandwidth is used by both Internet Service Providers, and Website Hosting Providers to track and limit the data that is transferred using their services. Here we will discuss the example of a website hosting provider.

All websites are limited to a specific amount of total bandwidth that is allowed to be transferred per month. Although some hosting companies offer "unlimited bandwidth", you should know: there is no such a thing. You will either be limited in file size transfer i.e. no way to upload file greater than 150kb or something similar.

Every file on your website has a specific file size. Let's say you have a webpage with a file size of 12kb and you have 3 images on that page, each of which have a file size of 20kb (3 x 20kb = 60kb). Let us also say that you have a .midi sound file, that plays automatically on your page which has a file size of 28kb; that is a total of 100kb transferred to your visitor's browser when they load your page. When that 100kb page is loaded 10 times into a visitor's browser, 1,000kb (10 x 100kb = 1,000kb) or 1MB of bandwidth will be used up. Every file that is transferred contributes to your bandwidth usage. Many providers will also include the amount of bandwidth used to transfer files to and from your site via FTP or a file management system into your total bandwidth calculation.

While making changes to a website often will not cause a huge increase in your bandwidth, if you are constantly changing large files on your site such as music, movies, or software downloads, this may significantly start to increase your used bandwidth. Images, movies, music and software files are the largest contributors to your bandwidth usage. You should try to limit the use of these types of files whenever possible. Also try to compress images and files to the smallest possible size. Bandwidth prices have been gradually dropping over the last few years, but while they are dropping, the average file size is increasing. This is causing a see-saw effect. The good news is if you optimize your graphics and compress your files, you can save your self hundreds, or even thousands of dollars as these prices drop.

In other words, this is really depending on how much multimedia (graphics, flash animation, background music, audio/video/software download etc.) you provide on your website as well as much people actively visit your site. A good estimate I usually do for my site is the following:
1. Total Page offered = TP
2. Total Media offered = TM
3. Average Page Size (including all images/flash/midi appearing on the page) = AP
4. Average Media Size = AM
5. Monthly Unique Visitors = V
6. Estimate Bandwidth Usage = B

B = [(TP * 0.3) * AP + (TM * 0.5) * AM] * V + (TM * 3)

What it basically says is that you can estimate your bandwidth requirement by assuming that each visitor views 30% of your site and 50% of your media; with an additional of approximately 3 times the amount of media you intend to offer for download. The 30% and 50% value will differ per website, but I find it on average that my visitors seems to like a few pages of my site and doesn't seem to visit the others as much, and as for the media, only about 1/2 of it gets actively viewed. Of course, if you get propel leeching (hotlinking your media), then it'd be a totally different story.

It only hurts your wallet a little bit when you buy too much bandwidth, but it hurts your wallet a lot if your business website goes down due to lack of bandwidth during a weekend and you don't notice it until beginning of business week. Typically, websites that are just starting out won't need more than 10 to 15GB a month. When you notice that your traffic grows, you can always upgrade to a bigger hosting plan.

Summary: Bandwidth usage is calculated based on file sizes and amount of traffic. Whether you're viewing, uploading or downloading files, you are using bandwidth. The less enhanced your site is - the less bandwidth it will consume.