Bandwidth refers to the amount of data that can be transmitted in a fixed amount of time. Data rates are usually measured in megabits (million bits) or megabytes (million bytes) per second. Many web hosts use both the terms bandwidth and data transfer interchangeably. Bandwidth is different from data transfer in the sense that bandwidth is how much data can be transferred at a time and data transfer is how much data is being transferred.
Different web site will need different amount of bandwidth. Bandwidth depends on the kind of content the site has. In order to calculate the amount of bandwidth needed by your site, always calculate the size of each Web Page multiplied by the number of web pages. Also the amount Internet users that access your site can also dramatically increase the bandwidth needed for your website.