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In the sciences, readability is a measure of an instrument's ability to display incremental changes in its output value. For example, a balance with a readability of 1 mg will not display any difference between objects with masses from 0.6 mg to 1.4 mg, because possible display values are 0 mg, 1 mg, 2 mg etc. Likewise, a balance with a readability of 0.1 mg will not display any difference between objects with masses from 0.06 mg to 0.14 mg.

Readability is defined as reading ease, especially as it results from a writing style. Extensive research has shown that easy-reading text improves comprehension, retention, reading speed, and reading persistence.

Ease-of-reading is the result of the interaction between the text and the reader. In the reader, those features affecting readability are 1. prior knowledge, 2. reading skill, 3. interest, and 4. motivation. In the text, those features are 1. content, 2. style, 3. design, and 4. structure (Gray and Leary 1935). The design can include the medium, layout, illustrations, reading and navigation aids, typeface, and color.

Among language experts, readability is a score produced by a readability formula. The formulas are widely used to match texts with the reading level of the audience. Extensive research has shown that the popular readability formulas are not 100% accurate, but they give a "good rough estimate" of the reading skill required to read a text. The readability formulas have greatly benefited millions of readers throughout the world in many languages. If there is any problem with the formulas, it is that they are not used enough (Chall 1958, DuBay 2006, Klare 1963).

Publishers not only use readability formulas to assess the reading level of a text. They also use word-frequency lists. The frequency of a word is a good indication of its ease-of-use. Text leveling, a subjective evaluation of a text based on training and experience, is another important adjunct of using a formula (Fry 2002).

Since the 1930s, national literacy surveys have shown that the average adult in the U.S. reads at the 8th-grade level. It is important to remember that one's level of education is no indication of one's reading skill. Many high-school graduates read at the 8th-grade level, college graduates at the 10th-grade level. With practice, readers with little formal education can often become advanced readers. (DuBay 2006, National Assessment of Adult Literacy).

Nearly all of today's blockbuster writers write at the 7th-grade level, including John Grisham, Stephen King, J. K. Rowling, and Dan Brown. Experts today recommend writing legal and health information at the 7th-grade level. Laws often require writing medical and safety information at the 5th-grade level (Doak, Doak, and Root 1996). Learning to write for a class of readers other than one's own is very difficult. It takes method, training, and lots of practice. As Jacques Barzun wrote, "Simple English is no person's native tongue."

Readability may be assessed by conducting a readability survey, or by application of readability tests which have been established through analysis of readability survey results. Often writers, editors and publishers make an intuitive assessment of readability based on experience, insight into their target audience, and knowledge of a number of rules of thumb, which are often derived from assessment of a number of readability survey results, such as that English textbooks are most readable when laid out with 20% whitespace on the page.

Re-readability - the propensity to again read something previously read after some time - appears to be a criterion dependent upon the reader. Yet, some authors, such as Robert A. Heinlein, appear to be more gifted to produce re-readable works than others as is apparent from the rate of re-printing.

In computer programming, readability refers to the ease with which a human reader can comprehend the purpose, flow, and operation of a machine-readable section of source code.

Readability is important because source code must inevitably be maintained by human beings, and if the code is not readable (or "obfuscated"), then bugs, inefficiencies, and duplicated code are likely to be introduced.

Many factors, having little or nothing to do with the ability of the computer to efficiently compile and execute the code, are therefore part of good programming style, because they affect readability. Some of these factors include:

• Different indentation styles (whitespace)
• Comments
• Decomposition
• Naming conventions for objects (such as variables, classes, procedures, etc)

Correct use of type size, line spacing, column width, text-color-background contrast and white space make text easy to read. See Typography#Readability and legibility for more details.

• Chall, J. S. 1958. Readability: An appraisal of research and application. Columbus, OH:Ohio State University Press.
• Doak, C. C., L. G. Doak, and J. H. Root. 1996. Teaching patients with low literacy skills. Philadelphia: J. P. Lippincott Company.
• DuBay, W. H. 2004. The Principles of Readability. A brief introduction to readability research.
• DuBay, W. H. 2006. Smart language: Readers, Readability, and the Grading of Text. Costa Mesa:Impact Information.
• Gray, W. S. and B. Leary. 1935. What makes a book readable. Chicago: Chicago University Press.
• Flesch, R. 1946. The art of plain talk. New York:Harpers.
• Fry, E. 2002. "Readability versus leveling." Reading teacher 56, no. 3:286-292.
• Klare, G. R. 1963. The measurement of readability. T. Ames, IA:Iowa State University Press.

• Readability test
• Gunning-Fog Index
• Flesch-Kincaid Readability Test
• SMOG Index
• Coleman-Liau Index
• Listenability