The Flesch/Flesch–Kincaid readability tests are readability tests designed to indicate comprehension difficulty when reading a passage of contemporary academic English. There are two tests, the Flesch Reading Ease, and the Flesch–Kincaid Grade Level. Although they use the same core measures (word length and sentence length), they have different weighting factors. The results of the two tests correlate approximately inversely: a text with a comparatively high score on the Reading Ease test should have a lower score on the Grade Level test. Rudolf Fleschdevised both systems while J. Peter Kincaid developed the latter for the United States Navy. Such readability tests suggest that many Wikipedia articles may be "too sophisticated" for their readers.^[1]

"The Flesch–Kincaid" (F–K) Reading grade level was developed under contract to the United States Navy in 1975 by J. Peter Kincaid and his team.^[2] Other related United States Navy research directed by Kincaid delved into high tech education (for example, the electronic authoring and delivery of technical information);^[3] usefulness of the Flesch–Kincaid readability formula;^[4] computer aids for editing tests;^[5] illustrated formats to teach procedures;^[6] and the Computer Readability Editing System (CRES).^[7]

The F-K formula was first used by the United States Army for assessing the difficulty of technical manuals in 1978 and soon after became the Department of Defense military standard. TheCommonwealth of Pennsylvania was the first state in the United States to require that automobile insurance policies be written at no higher than a ninth grade level (14 to 15 years of age) of reading difficulty, as measured by the F-K formula. This is now a common requirement in many other states and for other legal documents such as insurance policies.^[4]

In the Flesch Reading Ease test, higher scores indicate material that is easier to read; lower numbers mark passages that are more difficult to read. The formula for the Flesch Reading Ease Score (FRES) test is

Scores can be interpreted as shown in the table below.

Reader's Digest magazine has a readability index of about 65, Time magazine scores about 52, an average 6th grade student's (a 13-year-old) written assignment has a readability test of 60–70 (and a reading grade level of 6–7), and the Harvard Law Review has a general readability score in the low 30s. The highest (easiest) readability score possible is around 120 (e.g. every sentence consisting of only two one-syllable words). The score does not have a theoretical lower bound. It is possible to make the score as low as you want by arbitrarily including words with many syllables. This sentence, for example, taken as a reading passage unto itself, has a readability score of about thirty-three. The sentence, "The Australian platypus is seemingly a hybrid of a mammal and reptilian creature" is a 24.4 as it has 26 syllables and 13 words. One particularly long sentence about sharks in chapter 64 of Moby-Dick has a readability score of -146.77.^[9]

Many government agencies require documents or forms to meet specific readability levels.

The U.S. Department of Defense uses the Reading Ease test as the standard test of readability for its documents and forms.^[10] Florida requires that life insurance policies have a Flesch Reading Ease score of 45 or greater.^[11]

Use of this scale is so ubiquitous that it is bundled with popular word processing programs and services such as KWord, IBM Lotus Symphony, Microsoft Office Word, WordPerfect, andWordPro.

Polysyllabic words affect this score significantly more than they do the grade level score.

These readability tests are used extensively in the field of education. The "Flesch–Kincaid Grade Level Formula" translates the 0–100 score to a U.S. grade level, making it easier for teachers, parents, librarians, and others to judge the readability level of various books and texts. It can also mean the number of years of education generally required to understand this text, relevant when the formula results in a number greater than 10. The grade level is calculated with the following formula:

The result is a number that corresponds with a grade level. For example, a score of 8.2 would indicate that the text is expected to be understandable by an average student in year 8 in the United Kingdom or an 8th grade student in the United States. The sentence, "The Australian platypus is seemingly a hybrid of a mammal and reptilian creature" is a 13.1 as it has 26 syllables and 13 words.

The lowest grade level score in theory is −3.40, but there are few real passages in which every sentence consists of a single one-syllable word. Green Eggs and Ham by Dr. Seuss comes close, averaging 5.7 words per sentence and 1.02 syllables per word, with a grade level of −1.3. (Most of the 50 used words are monosyllabic; "anywhere", which occurs 8 times, is the only exception.)