A key factor in search engine optimization of websites is to provide reasonable content and to use the appropriate keywords with a natural frequence.
A rule of thumb says the main keyword should represent 2 to 5 % of the text on your webiste.
There are a lot of tools out there which take a website url as input and return several metrics also including the keyword density.
However, if you have to write your text before it is published on a website accessible for those tool, you need to keep an eye on on your keyword density upfront. To do this, you could use general word count facilities as provided by MS Word or others. But this requires you to count the keywords and calculate the density on your own. A more usable approach is to use a tool which directly returns the keyword density of the most frequently used words in your text.
There is a pretty simple but useful online tool provided by vexeo which you can use without any local installation: http://optimierung.vexeo.de/textoptimierung.html
Just put in your text, click the button and there you get your keyword density.