What is keyword density to start with, keyword density according to Wikipedia is:
“is the percentage of times a keyword or phrase appears on a web page compared to the total number of words on the page. In the context of search engine optimization keyword density can be used as a factor in determining whether a web page is relevant to a specified keyword or keyword phrase.”
Basically, a 5% keyword density in a 100 words article means that a certain keyword is mentioned 5 times on the article.
If we’re talking about the classic Search Engine Optimization (SEO) method, keyword density does exist. But nowadays, it was no longer a factor (speaking for Google placement). However, we should keep in mind that, a keyword appearing 5 times in a 100 word article is not ethical. It is as though you were only pleasing spiders rather than human readers – Keyword Stuffing. You can mention the keywords many times on an article but use synonyms or relevant keywords. If you really want Google to rank you well on Search Engine Result Page (SeRP), provide contextually relevant article. Believe me, if you provide contextually relevant article, 2-3 months time, your site maybe up to the first page of Google and in no. 1 position, of course given that you use targeted keywords.
So in summary, don’t worry about keyword density of your content, use related keywords instead and avoid words that has duplicate meaning. Write for readers and not spiders. Keyword density does not exist, yes, but Google gives more credit to sites that are user friendly. Don’t you think a keyword repeated 5 times in a 100 word article is a bit off, think about it; it does affect your readers.