In the optimum case, a website treated exactly one keyword or a combination of keyword and is also optimized for that keyword exactly. The frequency with which this keyword in the content of a web page appears, is one of the ranking factors for Google. As a concrete value to this pulls the so-called keyword density to rate the ratio between the number of occurrences of the keyword to the number of all words indicating on the website. There is no officially confirmed by Google value for a good keyword density. But are recommended for example in the book Website Boosting 2.0 by Mario Fischer (p. 311) values ​​between three and four percent. However, the computation of keyword Densitiy is already not without problems, because it is not known whether the complete text (including Boileplate code) is used on a Web page or as handled as stop words. Further information on this issue also provides Karl scratch with SEO Keyword Density myth .

As a target but you should definitely put that on the targeted keyword frequently of all the words (except stop words) occurs (and therefore also has the highest keyword density) so as an incorrect assessment of the theme of the site by search engines to prevent. It should however be noted that the keyword density is not too large, as this could indicate an attempt at manipulation. This is known as keyword stuffing and called contradicts the Google Webmaster Guidelines .

The keyword density is on one of the most important factors in the early days of information retrieval back, called Term Frequency so the frequency of occurrence of the term. If one now normalize the term frequency to a value between 0 and 1, then they would still be divided by the number of all words - and would thus arrived again at the definition of keyword density. All there is is still in the form of TF / IDF, and IDF for "Inverse Document Frequency" and is virtually the "value" of a word generally reduced when it occurs in many documents. Example: The word "not" occurs extremely often and is therefore suitable to classify, even if it is often on a website not a good document.

Due to this fact, it is likely that the keyword density is one of the more important onpage factors. This is supported among others by a series of tests of Sistrix, in which the relationship between density and keyword ranking was investigated. 

Keyword Density - Summary

The Keyword Density is the ratio of a word relative to all other words on a web page. The exact calculation of keyword density is not known, the main reason is that certain special cases such as plurals, compound words, stop words, etc. exist, their use must be determined experimentally. However, is certain that the keyword density has an influence on the ranking.

Finally, once the most important facts about keyword density in brief: