It predicts reading ease on a scale from 1 to 100, with 30 being "very difficult" and 70 being "easy." Flesch (p. 225) wrote that a score of 100 indicates reading matter understood by readers who have completed the fourth grade and are, in the language of the U.S. Census barely "functionally literate."

The second part of Flesch's formula predicts human interest by counting the number of personal words (such as pronouns and names) and personal sentences (such as quotes, exclamations, and incomplete sentences).

The formula for the updated Flesch Reading Ease score is:

Score = 206.835 - (1.015 x ASL) - (84.6 x ASW)

Where:

Score = position on a scale of 0 (difficult) to 100 (easy), with 30 = very difficult and 70 = suitable for adult audiences.

ASL = average sentence length (the number of words divided by the number of sentences).

ASW = average number of syllables per word (the number of syllables divided by the number of words).

This formula correlates .70 with the 1925 McCall-Crabbs reading tests and .64 with the 1950 version of the same tests.

In The Art of Readable Writing, Flesch (1949, p. 149), described his Reading Ease scale in this way:

Reading Ease Score Style Description Estimated Reading Grade Estimated Percent of U.S. Adults (1949)
0 to 30: Very Difficult College graduate 4.5
30 to 40: Difficult 13th to 16th grade 33
50 to 60: Fairly Difficult 10th to 12th grade 54
60 to 70: Standard 8th and 9th grade 83
70 to 80: Fairly Easy 7th grade 88
80 to 90: Easy 6th grade 91
90 to 100: Very Easy 5th grade 93
Table 3. Flesch's Reading Ease Scores

Flesch's Reading Ease formula became the most widely used formula and one of the most tested and reliable (Chall 1958, Klare 1963).

In an attempt to further simplify the Flesch Reading Ease formula, Farr, Jenkins, and Paterson (1951) substituted the average number of one-syllable words per hundred words for Flesh's syllable count. The modified formula is:

New Reading Ease score = 1.599 nosw - 1.015 sl - 31.517

Where:

nosw = number of one-syllable words per 100 words;

sl = average sentence length in words

This formula correlates better than .90 with the original Flesch Reading Ease formula and .70 with 75% comprehension of 100-word samplings of the McCall- Crabbs reading lessons. In 1976, a study commissioned by the U.S. Navy modified the Reading Ease formula to produce a grade-level score, This popular formula is known as the Flesch-Kincaid formula, the Flesch Grade-Scale formula or the Kincaid formula (See "The Navy Readability Indexes" below).