Article density checker is the best free online tool provided by small SEO tools. Density is a ratio of mass divde by voloume. By using this tool you can cheque the ratio of no of keywords by total numbers if words. By using this tool you may also cheque the no of % and position of word used in your article.  In seo, it's a very helpful tool to identify the density of article the higher density very helpfull and lower density does not help on optimization. Very easy to use just simple write article or paste and its show the word count and below the words and position with a percentage   
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