What's bandwidth or data transfer?

Bandwidth or Data Transfer
Very often web hosts talk about bandwidth and data transfer as the same entity. However, they are in fact, different.
Bandwidth is the amount of data that can be transferred at one time.
Data transfer is the actual amount of data transferred.
Think of it this way. If bandwidth were a tunnel, the bigger the tunnel the more cars can pass through it. Data transfer would then be the number of cars allowed to go through the tunnel in a given time period, say a month.
How it affects your website.
The less bandwidth you have, the slower it takes your website to load, regardless of your visitor's connection type. Visitors will have to wait their turn. The less data transfer you have, the more often you will find your website unavailable because you have reached the maximum amount of data allowed to be transferred.
How much do you need?
You first need to determine what is sufficient for your website to function. You will need to take into consideration the size of your website and the number of visitors you have in a month's time. Your goal is to ensure that all these visitors can view your website without any problem.
number of visitors expected
average page size (including all text, graphics, and other files)
expected page views by each visitor
Visitors x Avg. page size X Page views x 30 days = your monthly website data transfer
For example, your website consists of 5 pages, and each of these pages consists of text and images. Say, your average page size is 30KB. You expect 50 visitors per day, and each of these visitors will view all 5 pages on your website.
Your monthly website data transfer would then be:
50 visitors X 30KB X 5 pages X 30 days
= 225,000KB or 0.225GB/month