The common thought that keyword density should be 3%-5% and your word count should be considerable. Keyword density should only really be considered as part of your "anti-spam" portion of
SEO....you don't want to find yourself suddenly in the spam closet because you didn't check your density.
Another reason not to get to concerned with keyword density is simply there is not one tool that measures it the same. I use 4 -5 different tools (From Web CEO, Submit Express,
SEO Tool Box and others) and never get the same answer. Just keep the keyword percentage out of the attic and concetrate on writting good copy.
The reason the search engines consider this, is that in good natural writting your topic keywords fall in about the three- five percentile range
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There are apx 127 words in the above post and the word (keyword) appears 5 times.
that's about a 4% keyword density