Flesch Readability Formula

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Writing: Using Readability WriteItRead Rudolph Flesch

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The Flesch Reading Ease Formula and the Flesch-Kincaid Formula both use the Average Number of Words Per Sentence (Average Sentence Length) and Average Number of Syllables Per Word (Average Syllables per Word) as measures of readability.

Flesch Reading Ease

The Flesch Reading Ease Formula is:

Where ASL is the average sentence length (in words), and ASW is the average syllables per word. Alternatively, it can be written more clearly as:
fleschReadingEaseFormula2

The Flesch Reading Ease score is interpreted as follows:

Flesch Reading Ease Score	Readability Level	Education Level	Percentage Adults (Optimistic)
0 - 29	Very difficult	College Graduates	5%
30 - 49	Difficult	College	30%
50 - 59	Fairly difficult	Senior High School, A-level	50%
60 - 69	Plain English	13 to 15 year-olds	80%
70 - 79	Fairly easy	12 year-olds	90%
80 - 89	Easy	11 year-olds.
90 - 100	Very easy	10 year-olds

We can take the Flesch Reading Ease score to indicate roughly the percentage of the population able to read writing at this level. However, the above table is likely to be more appropriate to the population of the USA in the 1950's. See British American Comparison for equivalents to USA Grades.

Flesch-Kincaid Grade Level

The Flesch-Kincaid Grade Level Formula is:

Where ASL is the average sentence length in words, and ASW is the average syllable length of words (as above). A clearer version is:

The Flesch-Kincaid Grade Level uses USA Grades a comparison with the British system is here.

Sentence Length and Syllables per Word

Grade	Syllables/word	Average Sentence Length
Kindergarden	1.5	10
3-5	1.6	14
6-8	1.6	15
College/University	1.85	19 (14-23)

Publications

Publication	Reading Ease	ASL	ASW
Times	40-50	24	1.7
BBC	50-60	20	1.6
Sun	60-70	19	1.5
Sky	40-50	22	1.7

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