What is keyword density and what is the formula for knowing keyword density
 

From SEO point of view, keyword density will definitely help to stand out your content from others. The formula to know the keyword density is ( Total number of keyword/ total number of words in your article) multiply by 100.

Keyword density is a % of keyword appears on your webpage compared to the total no of words on the page.
Formula
(Total number of keyword/ total number of words in your article)*100

Keyword Density is to use the number of keywords in your Content. It helps to reach the goal of highest in search engine.

Keyword density: It is a percentage of the no: of times a keywords or phrases is present in the content of the webpage when compared to the total number of words present on the webpage. There is no universal regulation for the percentage of keyword density. Most of the SEO professional contemplate the keyword density from 1 to 3 percent as ideal. Simply overstuffing the webpage with keywords to gain an advantage in the SERPs will make the website appear like spam.

Total no: of keywords / Total no: of keywords on the webpage * 100

This is calculated as how often does a keyword appear in a website title or description. You do not want to go overboard with frequency, however, since on some engines if you repeat a word too many times, you are be penalized for "spamming" or keyword stuffing.

In general though, repeat your keyword in the document as many times as you can get away with, and up to 3-7 times in your list of metatag