Key word density is simply the relationship between the frequency of the usage of keywords and key phrases and the total word count.

Mathematically,  it can be expressed as  (The number of word/phrase repeats * the number of words in  the phrase  / total number of words on each page) * 100

What value does it have from an SEO perspective? Well, search engines have a desired keyword density of  3% to 5%. Anything over 5% may be frowned upon as ‘keyword stuffing’ and anything less than 3% is an indicator that your keywords do not have enough frequency.

How do you measure keyword density? I use the Firefox plugin for SEOQuake and what I particular like about this tool is the ability to import key words and phrases from Google allowing the key words to be modeled.  The way to use the information is to adjust your most important key words/phrases to fall in the 3% to 5% range – it is pretty impossible to do this for all of your keywords and have text which is readable for human beings.