A new study by scientists Antonio Gulli and Alessio Signorini estimates that the indexable World Wide Web currently contains about 11.5 billion web pages. The study also estimates the percentage coverage of the Internet by the four major search engines and the estimated intersection of the four indexes. The search engine coverages were calculated to be:

Google: 76.16%
Yahoo!: 69.32%
MSN: 61.90%
Ask Jeeves/Teoma: 57.62%

There is a difference between the ‘indexed Web’ and the ‘total Web’, with the indexable Web being defined as “the part of the Web which is considered for indexing by the major engines.” It is estimated that the number of web pages that can’t be reached by search engine crawlers is much higher than this. Examples of non-indexable web pages include secure or database driven pages.

The engines’ index intersection was about 28.85% or 2.7 billion pages. No wonder search results differ so much from engine to engine!

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