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The readability of a written text (not to be confused here with the legibility of handwriting) refers to the reading difficulty of a text or, conversely, the ease with which the text can be understood. As measured by several numerically based readability tests (or readability formulas), this generally refers to the educational or grade level required to understand the text. In this context, the word understand refers to the vocabulary and syntactical difficulty of the text rather than to the semantic complexity or depth of thought presented by the text.

Readability can be determined by direct testing on human readers, or it can be measured by means of any of several formulas which themselves are ultimately derived from testing on actual readers. Once a formula has been developed, it can thereafter be applied to any text simply and quickly.

Typically, the readability formulas rely on some combination of sentence length and the percentage of so-called difficult words. The latter can be defined in several different ways, such as the percentage of words having three or more syllables, or (indirectly) by the average number of syllables per word, or, in the case of some of the formulas (especially for younger readers), a list is used with difficult words being defined as any words not on the list.

Various software programs exist to measure readability and some word processing programs have a built-in readability function.


In Colonial America and up through the early part of the 19th century, young children, once they learned how to read (using, for example, the New England Primer, were then expected to graduate to reading adult-level reading material.

In the 1830s, William McGuffey developed one of the first sets of graded readers for younger school age children. His approach to doing this involved reading various articles to groups of young readers and then questioning them about their understanding of the subject matter and vocabulary. On the basis of their responses, he would classify the reading material in accord with his estimate of the difficulty of the piece. Continued testing and refining of the material eventually led to the progressively graded readers first published in 1837 as the McGuffey Readers.

Readability tests

At present there are over 200 readability formulas in existence.


The original Dale-Chall Readability Formula was first introduced in 1948.

Spache readability formula

The Spache readability formula was devised in 1953 primarily because of the inapplicability of then existing formulas when used for measuring readability of primary level reading material. It is similar to the Dale-Chall method in that it employs a word list, defining unfamiliar words (that is, words not on the word list) as being those which pupils in the 3rd grade or below would typically not recognize. The Spache formula is thus used to measure readability of texts thought to be appropriate for 3rd grade and below.

In its current incarnation, it is based on the Spache Revised Word List, 1974 ([1]). The basic method is to first compute the average sentence length (ASL) by dividing the number of words in a passage of approximately 100 to 150 words by the number of sentences in that same passage. Then, using the word list, the percentage of difficult words (PDW) is computed by counting the number of words not on the Spache list of familiar words and dividing that by the number of sentences and multiplying by 100 to get the percentage. There are special allowances for word variants, plurals, possessvies, etc.

The Spache Readability Index / Grade Level is then computed from the following formula:

Index = (0.141 x ASL) + (0.086 x PDW) + 0.839 (original formula)
Index = (0.121 x ASL) + (0.082 x PDW) + 0.659 (revised formula)


Gunning's Fog Index

Fry Graphical