With the decline of meta-tags, keyword density ranges have become very important. They’ve also become very controversial. Here’s the thing: you want a high enough keyword density—at least 7%–that your keywords rank highly in the bigger search engines, such as Google, Yahoo, DogPile, and HotBot.

But, as we discussed, you don’t want your keyword densities so high that they turn your content into over-hyped gobbledygook, nor do you want to raise a red flag when the spiders come crawling over your content. If your keyword density is 20% or more, the search engine will most likely red-flag you for “keyword stuffing” and penalize you by moving you down in the search results.

Thus, keyword density ranges are controversial. To make things worse, different search engines have different algorithms. One of them might thing an SEO keyword density of 18% is fine, another may not. The Google Adsense Empire Handbook