The Gunning’s Fog Index (or FOG) Readability Formula
Step 1: Take a sample passage of at least 100-words and count the number of exact words and sentences.
Step 2: Divide the total number of words in the sample by the number of sentences to arrive at
the Average Sentence Length (ASL).
Step 3: Count the number of words of three or more syllables
that are NOT (i) proper nouns, (ii) combinations of easy words or hyphenated words, or (iii) two-syllable verbs made into
three with -es and -ed endings.
Step 4: Divide this number by the number or words in the sample
passage. For example, 25 long words divided by 100 words gives you
25
Percent Hard Words (PHW).
Step 5: Add the ASL from Step 2 and the PHW
from Step 4.
Step 6: Multiply the result by 0.4.
The mathematical
formula is:
Grade Level = 0.4 (ASL
+ PHW)
where,
ASL = Average Sentence Length (i.e., number of words divided by the number of sentences)
PHW = Percentage of Hard Words
The underlying message of The Gunning Fog
Index formula is that short sentences written in Plain English achieve a better score than long sentences written in complicated
language.
The ideal score for readability with the Fog index is 7 or 8. Anything above 12 is too hard for most
people to read. For instance, The Bible, Shakespeare and Mark Twain have Fog Indexes of around 6. The leading magazines, like
Time, Newsweek, and the Wall Street Journal average around 11.
