[Help] Calculate properly the keyword density of a phrase of 2 or N words

I know to calculate a keyword density in an article its pretty much the (Keyword count)/(Total Words)

What I don't know is how to calculate the keyword density when the keyword is a phrase

Let's say (hypothetically for test uses) :

• I have an article with 100 words (and i use pure word with equal or more than 4 letters in each word so the 100 words will be included (since many words sometimes are ignore like the, and, in, at, a, I, etc... )
• I'm writing about "buying cheap cats food" as my keyword phrase or any other 4 words phrase
• my phrase appears 8 times in the article

What will be the exact formula to measure the phrase-keyword density?

if I use this site http://www.live-keyword-analysis.com/
it tell mes my keyword density is 8.2%

If i use this formula:
100*(Keyword count)/(Total Words)=8%

If i use the formulas that are set here (the last 2 formulas they use for phrases) http://www.vodahost.com/web-hosting-how-to-calculate-your-keyword-density.html/:
1st formula= Density = 100*(WordsInThePhrase * HowManyTimePhraseRepeats)/(Total Words)
= 100*(4*8)/100= 32% density

or this other formula they propose: Density = 100* ( HowManyTimePhraseRepeats / ( TotalWords -( HowManyTimePhraseRepeats x ( WordsInThePhrase-1 ) ) ) ) x 100

= 100*(8)/(100-(8*(4-1)))= 10.53%


So which is the formula to calculate properly the density of a keyword phrase on an article cause I'm quite mixed up ?

I don't get this
hope you can find the answer bro,
good luck.

Am also looking for this answer...

Ive been reading quite a lot and theres no damn right answer, many take it like the first option, many as the 2nd and many as the third
so im out of ideas, just wanted to do a better tool compared to http://www.live-keyword-analysis.com/ with a batch drag and drop for articles and get them analyzed

so if anyone has an idea of what should be done to correctly check an article for keyword phrase density please let me know