If you are launching a new site, calculating your bandwidth is difficult to determine. The best approach is to keep careful track of your website's activity for the first few months after it goes live so you can determine your actual monthly usage, and adjust the required bandwidth accordingly.
For more established sites, calculating your estimated bandwidth goes as follows:

1. Estimate the average page size of your site, in kilobytes (KB).
2. Multiply this value by the monthly average number of visitors.
3. Multiply the result from step 2. by the average number of pageviews per visitor.

Keep in mind that your calculations must include every one of your sites hosted on the server. If you run twelve domains on the same VPS account, take the time to add up the page sizes, visitors and pageviews across all domains.
These simple calculations should give you an idea of how much bandwidth you can expect to use, but some math is required. Basing your usage allotment solely on these rough estimates will not give you the entire picture.