".. Estimates of the size of the web are all over the place. I've heard 10 billion, 7 billion, 35 billion and 300 billion.
Much of the confusion emerges from how you define a webpage. Does it count if you have to use a form to get to the information? Does it count if you need a password? These are not trivial questions. It determines if you count every book listed in Amazon.com as a separate webpage (It is online though you do need to use a form) or if you count everything available in DialogWeb as a webpage (It is online though you do need your credit card and Dialog password). So, for example, do we count intranets? Do we count email messages sent to a mailing list that get archived?
Oh, and for not very good reasons, I'm currently guessing the web is between 15 and 20 billion webpages, not counting commercial or intranet or form-based data...
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David Novak, founder of the Spire Project
Novak, David. "10 Billion pages or more". Research Commentary. 2002. The Spire Project. 17 Nov 2003. <http://spireproject.com/art13.htm>.