it might be a good thing to know that bandwidth is something else than data transfer. Unfortunately most people, including hosts, mix up the two terms. I think you are talking about data transfer here ... the amount of data transferred between your account and visitors. For example, if a visitor dowloads a 10mb file, the data transfer is 10mb.
It is very hard to calculate data transfer. It all depends on the amount of visitors, the amount of pictures you use on your pages, how many pages a visitors visits, etc.