Capacity planning

A rough calculation to find out how much bandwidth your website would consume (and this will also decide what you will have to pay) -

Monthly Website Transfer = Number of visitors / expected number of visitors x Page size including the graphics of the page x Page views / expected pages viewed by each visitor x 30 days

If you offer downloads, then add the following:

Monthly Download Transfer = Average/Expected downloads x File Size x 30 days

Also add a small margin of error to take into account email traffic and your own uploads to the server.