Data transfer is often mistaken as bandwidth but the truth is they are different although very closely related.
Data transfer is the rate at which information travels across a node. You can imagine it as a street with traffic traveling on it in both directions (uploading and downloading). The wider the street, the more traffic can travel on it comfortably and conversely, the narrower the street, the slower traffic or data moves on it. While “data transfer” is the volume of traffic allowed, it is the consumption of bandwidth.
Both bandwidth and data transfer are in the same way important on the whole performance of your website, regardless of the connection speed of your online visitor. You should know how much you need for your websites especially if you intend to promote your website. In the lack of sufficient data transfer, your site might not be accessed if you reach your allocated monthly quota, meaning your site will not be online till another month begins.
To determine the sufficient amount of transfer rate for your website you should estimate the expected number of visitors who will view your site. Also check the page size including the graphics of the page and the expected pages viewed by each visitor. If you have an existing site you will find most of the information in your traffic history.
Then compute them using the formula below:
Visitors x Page size x Page views x 30 days = Monthly Website Transfer
If you offer downloads, then you should add the following:
Average/expected downloads x File Size x 30 days = Monthly Download Transfer
Source by Lloyd St. Clair