The Gunning Fog index is a commonly cited readability scoring formula.

The Gunning Fog formula generates a grade level, typically between 0 and 20. The formula estimates the years of formal education the reader requires to understand the text on first reading.

So, if a piece of text has a grade level readability score of 6 then this should be easily readable by those educated to 6th grade in the US schooling system, i.e. 11-12 year olds.

For more information, please read our blog: Readability and the Gunning Fog Index.
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