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Keyword difficulty is the difficulty for a web page to achieve top page positioning on a search engine such as Google for a keyword.

To quantify keyword difficulty, you will associate it with the keyword searches; the higher the keyword search volume, the more competitive the keyword, in most cases. We can look not only at the amount of search volume but also the number of indexed pages in Google containing the exact term. The higher the number of competing pages, the more the keyword would tend to be competitive.
Therefore, to give a mathematical definition, keyword difficulty can be approximated by the following formula:
Keyword Difficulty = Keyword Search Volume (Exact Match) x Google competing pages
In the above definition, it is obvious that if a certain keyword has a very high search volume and many competing pages, it is a difficult keyword for which to rank in Google.
The size of the domain can be measured by the number of indexed URLs in Google using the operator.