For the main article, see Readability
Readability tests, readability formulas, or readability metrics are formulae for evaluating the readability of text, usually by counting syllables, words, and sentences. Readability tests are often used as an alternative to conducting an actual statistical survey of human readers of the subject text (a readability survey). Word processing applications often have readability tests built-in, which can be deployed on documents in-editing.
The application of a useful readability test protocol will give a rough indication of a work's readability, with accuracy increasing when finding the average readability of a large number of works. The tests generate a score based on characteristics such as statistical average word length (which is used as an unreliable proxy for semantic difficulty) and sentence length (as an unreliable proxy for syntactic complexity) of the work.
Some readability formulas refer to a list of words graded for difficulty. These formulas attempt to overcome the fact that some words, like "television", are well known to younger children, but have many syllables. In practice, however, the utility of simple word and sentence length measures make them more popular for readability formulas. Scores are compared with scales based on judged linguistic difficulty or reading grade level. Many readability formulas measure word length in syllables rather than letters, but only SMOG has a computerized readability program incorporating an accurate syllable counter.
Since readability formulas do not directly take syntactic or semantic complexity into account, they are not considered definitive measures of readability.
Legibility is closely related to readability. ASTM D7298, Standard Test Method for Measurement of Comparative Legibility by Means of Polarizing Filter Instrumentation, is one test method for determining the ability to read printed material.
Readability tests 
- Accelerated Reader ATOS
- Automated Readability Index (ARI)
- Coleman-Liau Index
- Dale-Chall Readability Formula
- Flesch-Kincaid readability tests:
- Fry Readability Formula
- Gunning-Fog Index
- Lexile Framework for Reading
- Linsear Write
- Raygor Estimate Graph
- SMOG (Simple Measure Of Gobbledygook)
- Spache Readability Formula