Too often web hosts talk about bandwidth and data transfer in the same breath but truth be known they are different although very closely related. Bandwidth is how much data can be transferred at a time and data transfer is how much data is being transferred.
Think of it this way. If bandwidth were a bridge, then the bigger the bridge is the more vehicles can pass through it. While data transfer is the number of vehicles allowed on the bridge in say a month. In essence, data transfer is the consumption of bandwidth.How It Affects Your Site
The less bandwidth you have, the slower your site takes to load regardless of the visitor's connection type. If you have more visitors, some of them will have to wait their turn. The least data transfer you have, the more often you'll find your site unavailable because you're reached the maximum allowed until a new month rolls by or you upgrade your account.Determining Your Requirements
Usually when a host talks about bandwidth, they are referring to your transfer. So you need to figure out what is sufficient for your site to function. You'll need to gather some information; fairly easy if you already have a site. Most of this information is available from your traffic history. If you don't have an existing site, provide an optimistic estimate if you intend to heavily promote the site.
Then get ready for some math.
Find out the daily averages of: -
You should also throw in a small margin or error there to take into account email traffic and your own uploads to the server. If you offer downloads, then you should add the following:
Average/Expected downloads x File Size x 30 days = Monthly Download TransferUnlimited Plans
Bandwidth is very expensive. All hosts are limited by their own allocations. Thinking back to the bridge. What happens is each visitor to your site will be given a smaller lane to transfer the data, creating many tiny lanes therefore "unlimited". The more visitors you have the smaller each lane will be, which makes each visitor wait for the page to load.
More often than not there is little choice over your bandwidth as your host controls this. Some hosts may limit the number of simultaneous connections so in affect slowing down your site and refusing some visitors. This is called throttling. If you're concerned about this, you should ask the host how they control bandwidth usage or purchase a package with more data transfer. If you use HostVoice.net (link:http://hostvoice.net), this information is easily obtainable with one request.Reducing Transfers
Another good idea is to cache your website but you might want to set an expiry date in the HTTP headers so the browser will refresh the content after a certain time. Use mod-gzip. It could save you as much as 40% of your bandwidth. Out of control robots can also suck down your bandwidth like a black hole. So use robots.txt to keep spiders in check.