Actually, this can be a really difficult question to answer accurately. Basically, these are the things to consider when calculating your anticipated bandwidth usage for a given month (the following is a quote from an article on eHow.com that I think is very concise and accurate):
1
Determine the average size of each page on your website. If you've built the website yourself, use your web editor and look up the file sizes of each page you have built for your site. Add those file sizes together, then divide by the number of pages you have on your site to get the average page size, in KB (kilobytes).
2
Determine the number of page views you have per month. Ideally, you should do this after your website has been online for at least a week to get a reasonably accurate estimate of monthly traffic. Many web diagnostic tools, such as Google Analytics, can tell you the average number of page views you receive per day. Take an average of your daily page views and multiply by 30 to get an approximation of monthly traffic. If you can't go live with your site before calculating bandwidth, try to estimate based on the community or audience you're targeting, and be generous with your estimate.
3
Multiply the average page size by average page views per month to get the amount of bandwidth you'll use each month, in KB. Since bandwidth is usually provided in terms of GB (Gigabytes), divide your number of KB per month by 1,000,000 to get the rough number of GB of bandwidth per month that you'll need to buy. So if your average page size is 500 KB, and you think you'll get 2,000 page views per month, you know that you'll need to pay for at least 500x2,000 KB per month, which is 1,000,000 KB or 1GB per month.
I hope this helps.
