The Flesch Grade Level Readability Formula
The Flesch Grade Level Readability Formula is a way of calculating the appropriateness of a text for a particular reading ability. Microsoft Word can be used to calculate this (see picture below).
The formula for the Flesch-Kincaid Grade Level score is:
(0.39 x ASL) + (11.8 x ASW) – 15.59
where ASL is the average sentence length and ASW is the average number of syllables per word.
The score indicates the grade-school level: e.g. a score of 5.3 means that an average fifth grader (a Year Six child) would be able to read the text. (I would add 5 to the score to get a reading age, so a score of 5.3 would mean a reading age of 10.3).
It’s far from perfect. Sentence length massively affects the score, so fairly simple compound sentences will increase the score, especially in short texts. And the number of syllables in a word is far from a perfect indicator of word difficulty. But as long as you recognise this, it’s a useful, rough tool.