Fog Index

// Disclaimer:: Fog Index is not developed by me. I am just presenting it in my words. :)

Fog Index measures the complexity of writing samples. It provides a means of calculating the educational level (in years) required to understand a particular passage.

To calculate Fog Index of a passage, follow the following steps:-
Count the number of words in a paragraph/passage
Count number of sentences.
Calculate Average Sentence Length by dividing Number of Words by Number of Sentence
Count the Number of Long or Hard Words (more than three syllables). Divide this number by total number of words in the passage. Multiply this number with 100. This will give you Percentage of Hard Words in the passage/paragraph.
Add the Average Sentence Length to the Percentage of Hard Words. Multiply the outcome with 0.4. The number you get is Fog Index. This number indicates the number of years of education required to easily understand the passage

Suppose, in a passage,
Number of Words = 126
Number of Sentence = 9
Average Sentence Length = 126/9 = 14
Number of Hard Words = 13
% Hard Words = 13/126 * 100 = 10.314
Fog Index = (Average Sentence Length + % Hard Words) * 0.4 = (14 + 10.314) = 9.72.

So, one needs an education spanning a little less than 10 years to understand this passage.