Use SEOquake firefox addons. It's free, simple and will give you right results
The idea is most tools divide the keyword on the number of words which is incorrect. If you mention you 3 words keywords in 900 word article 3 times ... this would 1% density. 3 mentions / 900 words / number of words in the keyword ( 3 )
Same for 2 words, then you divide the number of words in article by 2
Short story long, use the addon, it knows that