Keyword density is an indicator of the number of times the selected keyword appears in the web page.
Keyword density is always expressed as a percentage of the total word content on a given web page.
Suppose you have 100 words on your webpage (not including HMTL code used for writing the web page), and you use a certain keyword for five times in the content. The keyword density on that page is got by simply dividing the total number of keywords, by the total number of words that appear on your web page. So here it is 5 divided by 100 = .05. Because keyword density is a percentage of the total word count on the page, multiply the above by 100, that is 0.05 x 100 = 5%
The accepted standard for a keyword density is between 3% and 5%, to get recognized by the search engines and you should never exceed it.
Remember, that this rule applies to every page on your site. It also applies to not just to one keyword but also a set of keywords that relates to a different product or service. The keyword density should always be between 3% and 5%.
Check your keyword density with this free tool. It helps you to determine the optimum keyword density percentage: