Keyword density can be explained as percentage of the keyword repetition to the total number of words present in that particular web page. It is one of the major factors which have huge influence in a web site ranking in the search engines. There is no exact formula for keyword density and it should normally be around the 2-5%.
The average keyword density should be 3% for each article. To find out how much keywords to use, you times 3% with the total word count of the article. For example, if your article is 500 words, the total word count for the keywords which you should use is 25 (500 x 5/100)
The higher the density of your keywords the more relevant your page will be. Typically, for a good structure you can try to have 3 to 7% for major keywords. And for minor keywords you can have a density of 1 to 2%.
When you use the Google Keyword Tool, enter your main keyword phrase and then every other phrase that comes up with it are your LSI (latent semantic indexing) keywords. In other words, they are words related to your main keyword and you should use a few of them since Google appreciates it, because you're not 'spamming' the keyword.
Keyword density is how many time call keyword in a page and keyword length in % call keywords density.
There are many free resources whish will help you check the keyword density of your page. Just Google it..you will get lots of options.
Keyword density is important and should be maintained in 3%-5% so google will not mark your site as spam.
Keyword density is important because search engines use this information to categorize a site's theme, and to determine which terms the site is relevant to. The perfect keyword density will help achieve higher search engine positions. Keyword density needs to be balanced correctly too low and you will not get the optimum benefit, too high and your page might get flagged for “keyword spamming. The keyword density tool is useful for helping webmasters and SEOs achieve their optimum keyword density for a set of key terms.
I wish people who don't know what they're talking about would just admit it and not reply ... it confuses people. Keyword density has nothing to do with ranking position. Read my article on Keyword Density for a fuller explanation. Keyword placement however does play a very important role. You should have your keyphrases in the title tag, H1 tags and at least once in a natural sounding manner on the page text.