A keyword is a word or phrase that closely describes the topic of a document. For instance, if a web page is devoted to African lions, then lion, lions, African lions,and African lion would all likely be designated as keywords. Keyword density is the percentage of words in a document, article, or web page, that are keywords. In the earlier days of the Internet, search engines primarily used keyword density to rank pages about a subject, leading many to pepper web page text with as many keywords as possible.
To discover the keyword density of a page or article, divide the total number of words by the amount of times the keyword is used. If the keyword lions appeared 10 times in an article of 100 words, the keyword density would be described as 10%. The formula for keyword phrases can be somewhat more tricky, because instead of counting the phrase as one term, it is counted by individual words. Thus, if the term African lions appears 10 times in a 100 word article, it could be considered by some search engines to have a density of 20% rather than 10%.
Because web pages or articles with an extremely high keyword density are often considered to be spam, some search filters will penalize the ranking of a page if the density is too high. Generally, most search engines seem to allow a maximum of about 5% keyword, although this varies somewhat between different companies. Using dozens of keywords to try to boost ratings is often referred to as keyword stuffing, and should be avoided due to stuffing penalties.
Another reason to keep keyword density relatively low is to create aesthetically pleasing and easily readable material. According to many works on grammar and style in the English language, writers should take care to avoid using the same word or phrase within the same 10 words. Adhering to this simple rule may help prevent the overuse of keywords.
Although keywords still play a part in search engine operations, they are no longer the primary source of page ranking. Some search engines work like an advertising agency, and charge customers for high rankings. Others use the popularity of the site to determine search ranking, and will award the highest places to websites that post the most hits in a given period.
If the mathematics of word counting is too difficult or time-consuming, the keyword density can be determined by using online tools. Several websites offer keyword density calculators that will run the math quickly and provide the answer as a percentage. Additionally, some word processing programs may have an option to search for keyword frequency. When writing quickly, plugging text into one of these calculating tools can help ensure that density is at a level that is useful to attract attention but not overwhelm search engines or site visitors.