A story text is “decodable” only if ALL the speech sound-letter correspondences in ALL the words in that text have been taught prior to the introduction of the text to the student.  This means that reading programs with “decodable texts” have systematically and explicitly taught all the speech sound-letter correspondences necessary to decode (read) words in them BEFORE the child encounters these words in a text.

For example, to properly read the word cat, the child must have been taught each of the three speech sound-letter correspondences: c = /k/;  a = /a/;  and t = /t/.  In laundry, there are six speech sounds and combinations that the child must be prepared to decode.  For laundry to become a decodable word, the child would have to have had instruction in all six speech sound-letter combinations:  l - /l/;  au = /o/;  n = /n/;  d = /d/;  r = /r/;  and  y = /?/.   Having had instruction in fewer than six speech sound-letter correspondences would make laundry “nondecodable.”

Total the number of decodable words in each story.  Remember, if any speech sound-letter correspondences in a word have not been taught previously, that word is not decodable.  A child may have been taught /m/ = m  and “silent e,” but knowing those two speech sound-letter correspondences does not mean “mouse” is decodable because the /ou/ and /s/ speech sound-letter correspondences have not been taught.

Total the number of words in the story. Divide the number of decodable words by the total number of words in the story. The result is the percentage of decodable words in that story.

For example, if a story contains 34 words and two of those words have had all of their speech sound-letter correspondences taught, divide 2 by 34 for 5.9%.  The percentage of “decodable” words is 5.9%.  That is, only 5.9% of speech sound-letter correspondences in those 34 words were taught before the child encountered them in print.