The tools behind Readability.info analyzes the characteristics of your writing and outputs a variety of readability scores and statistics based on common readability formulas. The underlying tool is the GNU style utility. You can also see readability scores for popular Web sites to help get an idea of how they stack up.

Readability grades

Kincaid formula
The Kincaid Formula has been developed for Navy training manuals, that ranged in difficulty from 5.5 to 16.3. It is probably best applied to technical documents, because it is based on adult training manuals rather than school book text. Dialogs (often found in fictional texts) are usually a series of short sentences, which lowers the score. On the other hand, scientific texts with many long scientific terms are rated higher, although they are not necessarily harder to read for people who are familiar with those terms.

Kincaid = 11.8*syllables/wds+0.39*wds/sentences-15.59

Automated Readability Index
The Automated Readability Index is typically higher than Kincaid and Coleman-Liau, but lower than Flesch.

ARI = 4.71*chars/wds+0.5*wds/sentences-21.43

Coleman-Liau Formula
The Coleman-Liau Formula usually gives a lower grade than Kincaid, ARI and Flesch when applied to technical documents.

Coleman-Liau = 5.89*chars/wds-0.3*sentences/(100*wds)-15.8

Flesh reading easy formula
The Flesh reading easy formula has been developed by Flesh in 1948 and it is based on school text covering grade 3 to 12. It is wide spread, especially in the USA, because of good results and simple computation. The index is usually between 0 (hard) and 100 (easy), standard English documents averages approximately 60 to 70. Applying it to German documents does not deliver good results because of the different language structure.

Flesch Index = 206.835-84.6*syll/wds-1.015*wds/sent

Fog Index
The Fog index has been developed by Robert Gunning. Its value is a school grade. The ``ideal'' Fog Index level is 7 or 8. A level above 12 indicates the writing sample is too hard for most people to read. Only use it on texts of at least hundred words to get meaningful results. Note that a correct implementation would not count words of three or more syllables that are proper names, combinations of easy words, or made three syllables by suffixes such as -ed, -es, or -ing.

Fog Index = 0.4*(wds/sent+100*((wds >= 3 syll)/wds))

Lix formula
The Lix formula developed by Bjornsson from Sweden is very simple and employs a mapping table as well:

Lix = wds/sent+100*(wds >= 6 char)/wds

SMOG-Grading
The SMOG-Grading for English texts has been developed by McLaughlin in 1969. Its result is a school grade.

SMOG-Grading = square root of (((wds >= 3 syll)/sent)*30) + 3

Word usage

The word usage counts are intended to help identify excessive use of particular parts of speech.
Verb Phrases
The category of verbs labeled "to be" identifies phrases using the passive voice. Use the passive voice sparingly, in favor of more direct verb forms. The flag -p causes style to list all occurrences of the passive voice.

The verb category "aux" measures the use of modal auxiliary verbs, such as "can", "could", and "should". Modal auxiliary verbs modify the mood of a verb.

Conjunctions
The conjunctions counted by style are coordinating and subordinating. Coordinating conjunctions join grammatically equal sentence fragments, such as a noun with a noun, a phrase with a phrase, or a clause to a clause. Coordinating conjunctions are "and," "but," "or," "yet," and "nor."

Subordinating conjunctions connect clauses of unequal status. A subordinating conjunction links a subordinate clause, which is unable to stand alone, to an independent clause. Examples of subordinating conjunctions are "because," "although," and "even if."

Pronouns
Pronouns are contextual references to nouns and noun phrases. Documents with few pronouns generally lack cohesiveness and fluidity. Too many pronouns may indicate ambiguity.

Nominalizations
Nominalizations are verbs that are changed to nouns. Style recognizes words that end in "ment," "ance," "ence," or "ion" as nominalizations. Examples are "endowment," "admittance," and "nominalization." Too much nominalization in a document can sound abstract and be difficult to understand.
Scores for some popular Web sites.