I do not recommend spending too much time worrying about keyword density. Yes, repeat the main keyword phrase a couple times in the content, and have it in the title and also in one or more image alt tags, but more is -- most of the time -- unnecessary. Being 100% right on the money with keyword density will increase your ranking slightly, but other things, like links, have a much bigger impact and take less time to get right.
Only if you are in a very competitive industry where every little bit of boost in ranking makes a difference is it worth spending your time tweaking keyword density. The density should be between 2% and 7%, but the exact optimum density varies from one search engine to the next. If you optimize the density for one search engine -- that is, if you are able to find the exact density to gain ranking in the SERPs of one search engine -- chances are that it will NOT dramatically affect your ranking and, worse, it could hurt your ranking on another search engine.
Competitive Intelligence and competitor analysis resources, tools and services for organic and paid PPC search activities, traffic, keyword bidding, ranking and more. Also see my list of Keyword Research tools.
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