Dale–Chall readability formula
The Dale–Chall readability formula is a readability test that provides a numeric gauge of the comprehension difficulty that readers come upon when reading a text. It uses a list of 3000 words that groups of fourth-grade American students could reliably understand, considering any word not on that list to be difficult.

History
The formula was inspired by Rudolf Flesch's Flesch–Kincaid readability test which used word-length to determine how difficult a word was for readers to understand. Edgar Dale and Jeanne Chall instead used a list of 763 words that 80% of fourth-grade students were familiar with, such as "no", "yes", and other such very basic words to determine which words were difficult. The Dale-Chall Readability Formula was originally published in their 1948 article A Formula for Predicting Readability and updated in 1995 in Readability Revisited: The New Dale-Chall Readability Formula, which expanded the word list to 3,000 familiar words (text file version).

Formula
The formula for calculating the raw score of the Dale–Chall readability score (1948) is given below:

If the percentage of difficult words is above 5%, then add 3.6365 to the raw score to get the adjusted score, otherwise the adjusted score is equal to the raw score. Difficult words are all words that are not on the word list, but it has to be considered that the word list contains the basic forms of e.g. verbs and nouns. Regular plurals of nouns, regular past tense forms, progressive forms of verbs etc. have to be added.

See also
Readability

References

Readability tests