(1) Flesch-Kincaid Reading Ease

The Flesch-Kincaid Reading Ease index predicts how easy/difficult the text will be for others to read based on average sentence length and word length. The index values range from zero (0) to one-hundred (100), zero being most difficult and one-hundred least difficult. The index maps into grade levels also as follows:

90-100 Very Easy (4th Grade)

80-89 Easy (5th Grade)

70-79 Fairly Easy (6th Grade)

60-69 Standard (7th-8th Grade)

50-59 Fairly Hard (Some High School)

40-49 Difficult (High School - College)

30-39 Very Difficult (College and Beyond)

(2) Flesch Grade Level

The Flesch-Kincaid Grade Level is an index intended to represent the level of linguistic maturity reflected in the text. The inputs to its formula are sentence and word lengths. The index is reported as a standard grade level.

(3) Gunning Fog Index

Gunning's Fog index uses average sentence length and big words (the number of words containing four syllables or more) as a measure of reading difficulty. Low numbers, say, below ten (10), signify the use of simple vocabulary, while higher numbers, e.g., in the twenties, signal the presence of verbose text.


The flags reported relate to the so-called terminal unit [T_Unit]. Terminal units are constructions in the text capable of standing as independent sentences in their own right. There are five flags in this class, plus a composite index derived from two of the flag values:

    [WT=Value] -- Mean words per T_Unit

    [WC=Value] -- Mean words per clause

    [WS=Value] -- Mean words per sentence

    [CT=Value] -- Mean clauses per T_Unit

    [TS=Value] -- Mean T_Units per sentence

    T_Unit Composite = TS*WT

Each of these flags contains a number relating to the syntactic complexity of sentences. The composite index ranges from around ten (least complex) to above twenty (most complex).