Like the ideal content length, the best keyword density is... Many people have posed this question, but only a few people have answered it. Because it all relies on the issue you're writing about, there is no exact solution to this question. Some themes are well-suited to long-form writing with a large number of related keywords and synonyms. On the other side, there are some topics that benefit from a short piece of material with a lot of repetition of the same keywords. The best advice on this subject is to write for humans rather than computers algorithms and crawlers.