Keyword density is the number of times a keyword is used on a web page in proportion with other words. You can calculate the keyword density of a particular text as:
keyword count/ total word count * 100.
Optimal keyword density ensures that your webpage will be ranked well for the targeted keywords.
As such, there is no fixed keyword density for any search engine. The idea is to use them in such a clever fashion that neither does it hinder readability or dilute the real message (as in the case of keyword stuffing), nor is it a case of under-utilization so that you don’t rank high on search results.