The keyword density tool is useful for helping webmasters and SEOs achieve their optimum keyword density for a set of key terms.
Keyword density is important because search engines use this information to categorize a site's theme, and to determine which terms the site is relevant to. The perfect keyword density will help achieve higher search engine positions. Keyword density needs to be balanced correctly (too low and you will not get the optimum benefit, too high and your page might get flagged for “keyword spamming”).

This tool will analyze your chosen URL and return a table of keyword density values for one-, two-, or three-word key terms. In an attempt to mimic the function of search engine spiders, it will filter out common stop words (since these will probably be ignored by search engines). It will avoid filtering out stop words in the middle of a term, however (for example: “designing with CSS” would go through, even though “with” is a stop word).